The short version is mostly a mixture of random chance + infinitesimally small sampling time (with respect to geologic time) + vastly different relevant areal extent of the regions being compared.

For the longer answer, let's first take a look at the referenced map here (note this is a live map, so it may not reflect OPs question if viewed later in time). At the time being asked, it does indeed show that there are several volcanoes in Central and South America that are in some stage of eruption and/or unrest, but it's also worth noting that the original premise as stated is wrong from the start, i.e., there is at least one volcano in similar states of unrest in North America unless we exclude Mexico from the traditional definition of North America (not to mention various eruptions throughout the Aleutians and Hawaii, which are both part of North America as well, but would not be part considered here because of the arbitrary limits placed by only looking at "mainland" North America). It's also worth noting on this map, that in North, Central, and South America, there are consistently many more volcanoes not in a state of unrest, so even in Central and South America, the currently active volcanoes represent a small proportion of the total separate volcanic systems.

Next, let's consider the dominant driver of much of the volcanism along the North-Central-South American west coast, i.e., subduction related arc volcanism. If we consider the portions of these coasts that are still active subduction zones, e.g., this map, we can see that (especially if we exclude the Aleutians), a much smaller proportion of the North American west coast contain subduction zones compared to nearly all of Central and South America containing active subduction zones. While there is (geologically) recent volcanism in the southwestern US and northern Mexico, much of this related to other processes or the past history of subduction in these regions, but in general, we broadly expect the activity of many of these systems to be substantially less than active arc volcanism. Thus, in a simple sense, if we assumed that (1) there is broadly more active volcanism in arc settings above subduction zones compared to other volcanic settings within the broad area of interest and (2) over a given distance of arc we'd expect some number of volcanoes to be erupting at any given time - these two together, along with the relatively small amount of arc "real estate" in mainland North America as defined, would predict that generally the probability of seeing activity at any given time in North America is much less than in Central and South America simply because they're bigger.

Finally, time is a critical factor here, and specifically the effectively instantaneous nature of the observation with which we've started. If we consider the history of eruptions in the Cascades, e.g., this graphic, we can see that in the still very recent past the Cascades have been quite (and mostly consistently) active. Now, it is true that in the last ~100 years, the Cascades have largely been quiet (with the exception of the Mt. Saint Helen's eruption). The question of why the Cascades seem to be less active compared to many other arcs is directly discussed in this write up from volcanologist Erik Klemetti, in which he muses on potential geologic/tectonic reasons for the tendency (e.g., differences in the angle of the slab and/or age of the oceanic crust being subducted), but ultimately largely argues for the same thing I'm suggesting here, i.e., random chance and extremely short time sampling of very slow processes.

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Maybe worth pointing out that the Nazca Plate, which is the tectonic plate subducting under nearly the entire Pacific margin of South America, is also the fastest moving plate on the planet. Map here. Does this have anything to do with the subsequent amount of volcanic activity in the Andes? In my head it makes sense that more subducting material=more partial melting=more eruptions, but I don't know for sure.

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Neat! Thanks for the excellent explanation. Do you think that the Gorda/ Juan de Fuca plates being locked solid might be creating a delayed onset of volcanic activity? Like we know the last mega thrust quake was 200-ish years ago, could it be that the crust pushed down in that event just hasn’t had time to really melt much yet?

Awesome analysis and answer, but isn't Hawaii firmly situated on the pacific plate? I don't think it's considered part of North America

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North America geographically is not the same as North America tectonically. Hawaii is on the Pacific plate, but as part of the US, geographically it is often considered as part of North America (though definitions vary). Similarly, the North American Plate contains portions of Russia (which geographically tends to not be considered part of North America), but does not contain the Caribbean (which does tend to be considered part of North America geographically). In my answer, I assumed OP was using the term North America in a geographic context.

Again, I would start with questioning the validity of the underlying premise, i.e., considering extremely short duration records of activity is not a valid way to compare volcanic arcs. A more valid comparison would be things like total magmatic productivity (including both extrusive and intrusive) per unit arc length, total eruptive products per unit arc length, or similar. The question then becomes whether, when viewed in a valid way, whether the Andes are more magmatically productive than the Cascades. I don't know the answer to this (and it's not readily apparent from a quick glance at the literature), but that's really the first step, but as we'll see, an inherently challenging step.

With respect to the proposed hypothesis, i.e., that subduction rate directly ties to volcanic production rate, while logical this ends up being problematic, or at least not simple to demonstrate. It does have some support in the literature, i.e., there are studies that find relationships between estimates of volcanic activity and subduction rate (e.g., Huang & Lundstrom, 2007, Syracuse & Abers, 2006 - note for this last one, the Cascades are not even included in the compilation), but this correlation breaks down in other studies (e.g., Acocello & Funiciello, 2010). There are a variety of other things that have been argued to influence volcanic productivity, e.g., instead of the rate of subduction, the degree of obliquity of subduction (e.g., Sheldrake et al., 2020, Gazel et al., 2021), regional stress state of the overriding plate (e.g., Takada, 1994) - which is not wholly independent of the degree of obliquity or subduction rate, or the extent of hydration of the subducting slab (e.g., Till et al., 2019, Cooper et al., 2020) - which itself might correlate to things like slab age which in turn could correlate to subduction rate, amongst other controls. As highlighted in several of these papers (most prominently Acocello & Funiciello and Till et al) though, working out any of these controls is problematic because of a variety of challenges that mirror the issues mentioned in the first paragraph. For example Acocello & Funiciello suggest that the degree to which magmatic activity will or will not correlate with subduction rate depends on the time frames, i.e., plate rates are often estimated over long time frames than magmatic activity and whether you see a correlation or not depends on whether you take a long or short term view of magmatic activity. They also touch on the inherent issue with defining magmatic productivity. This is really picked up on in Till et al. where they discuss that (1) estimates of surface eruptive volumes are often highly uncertain and (2) estimates of intrusive volumes are even more uncertain. Importantly, apparent temporal or spatial changes in magmatic productivity based mostly on surface volumes may largely reflect changes in the intrusive vs extrusive ratio (which might depend on things like stress state in terms of whether magma is able to reach the surface or not, etc). So, essentially, we are still stuck with inherent challenges in truly assessing whether a particular arc is more productive than another. Suffice to say and as originally stated, one thing is for sure, specifically that an instantaneous snapshot of volcanic activity is definitely not the appropriate way to compare volcanic activity.

>Do you think that the Gorda/ Juan de Fuca plates being locked solid

This is not an accurate depiction of this margin though. Cascadia appears to experience both large slip events, but also so-called episodic tremor and slip (ETS) events (e.g., Dragert et al., 2001, Brudzinki & Allen, 2007), so it's not as though subduction is completely paused along this margin between megathrust events.

> might be creating a delayed onset of volcanic activity? Like we know the last mega thrust quake was 200-ish years ago, could it be that the crust pushed down in that event just hasn’t had time to really melt much yet?

More importantly though, and similar to the original consideration of OP, this is a ridiculously short time frame to consider when we're thinking about processes that when discussing the relevant rates we typically average over 10^(4) to 10^(6) years.

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Thanks! Neat, that Funicello paper lists the Cascades as having (within error) the same volcanic output per 100 km arc length as southern Chile. Also surprisingly, all the arcs listed have roughly the same volcanic output within one order of magnitude.