> The insane popularity of channels such as 3Blue1Brown suggests that a lot of people do.

I don't think this is true.

First of all, the fact that watching videos is not a sufficient way to learn math does not mean that videos cannot be an excellent supplement for learning math. 3B1B's linear algebra videos for example are wonderful tools for gaining geometric understanding and motivation behind why things are the way the are in linear algebra, but of course they won't teach you how to invert a matrix. (He even says something to this effect explicitly many times throughout the series, if I recall.) Not everyone who watches those videos is engaging shallowly with math just because they're watching videos about it, there's plenty of value for serious students as well.

Your next argument might be that the sheer number of views indicates an unrealistic volume of serious students. This is true, but it still doesn't indicate that people think watching videos is a good way to learn math; in particular, nothing says they're watching those videos because they're trying to learn math. They're beautiful and interesting and well put together videos, and I'm sure many people enjoy watching them passively fully conscious of the fact that they aren't actually deeply learning math. I don't think there's anything wrong with that!

I'm sure there is a subset of viewers who think they're actually learning math by merely watching videos, but I'm willing to bet it's a relatively small fraction.