Storage units are definitely getting filled up at a wild rate or developers would stop building them. They do a lot of research on the market and look at average household income and determine if there's a market for it. Americans buy a lot of stuff, gotta put it somewhere. The way I see it, small houses can't hold a lot of stuff, where are you going to keep your Christmas decorations? Where does your lawnmower go in the winter if you don't have a garage? Snowblower in the warmer months? We buy big things that we don't need for portions of the year, gotta put them somewhere.

On the mattress front, mattresses are super expensive. A store needs to sell between 5-10 a month to pay rent. That's some pretty low pressure sales considering when someone needs a mattress they really need a mattress. Landlords also love mattress stores (chain ones like mattress warehouse and mattress firm, local ones are typically for shittier centers, think something like Cromwell Field out in Glen Burnie) since they have great credit and require very little parking meaning the shopping center can focus it's parking on grocery store parking etc. It's a great business model so long as you don't get greedy like mattress firm did a few years back. Interesting story if you want to read about it look up mattress firm corruption or something like that.

Commercial real estate broker here. Self storage is one of the hottest asset classes in the entire country right now. Things absolutely print money. Think about it, pretty minimal investment cost since you're effectively building a concrete box with some cages in it. Minimal payroll costs since you don't need more than two people max to keep the thing running. High rents per storage unit and they're pretty much guaranteed to fill up. Once one is stabilized (like 90% full) the developer sells it at a ~5% cap rate (cap rate is Net Operating Income/property value). The higher the NOI and the lower the purchase price when you bought the property, the more you can sell it for. Developers are making a killing on these things and have been for a few years now.

Mattresses cost like 50-100 bucks to make, the profit margins on them are insane.

I have no idea how psychics make money. Preying on the insecure I guess but I've never seen someone go into one.