Where there's smoke there's fire (i.e. many people saying the same thing needs to be explained). Arguments like "surely that many people can't be wrong" or "they came to their conclusions mostly independently" are often implied but not stated. To refute those, learning about the phenomenon of information cascades is very helpful; it explains how large fractions of the population can end up believing something based on very little evidence.

The tl;dr is that if, for a particular decision, only the decision is visible and not the detailed reasoning/evidence/information, and a large majority values "social proof" or conformity more than their private information, then their private information doesn't get incorporated into the public set of information, so only the very early people who decide first get to define that public set. It's a very "sticky" process. For example, consider people in a crowd deciding whether to panic: One or two people possibly saw something concerning and screamed or ran; then, later people react less to the actual seen thing (or lack of it), and more to the number of other people reacting or not reacting. The early or closest people get to set the "tone" - if there's no reaction from others, you infer it was not worth panicking after all, and don't join, strengthening that impression; if there is, you infer it was, and join in. That can easily end up causing panics out of nowhere, or not causing panics when you'd expect them.

Once you know this, it's easy to see how millions of people can be wrong, and how the "wisdom of crowds" fails to work in such cases. Then, however, you also need to make sure the scientists themselves don't suffer from an information cascade, and they usually do - they didn't all arrive at their opinions independently from scratch. New information being available and undoing a wrong cascade partially explains Kuhn's paradigm shifts. Even in the hard sciences, the social environment can become so bad that scientists conform to fashions when they shouldn't, for no good reason - it's why plate tectonics took so long to dislodge earlier shittier theories despite the strong evidence, for example.

So there's no real way to tell which ideas like homeopathy do or don't work based solely on judging their disparity in popularity among different crowds (at least not without risking being fooled that way); you have to reason about them, at least a little bit. Or trust that experts will usually get it right regardless, which is reasonable, but not foolproof.