They don't necessarily. e.g. I'd consider Ravi Vakil's Algebraic Geometry videos[0] among the highest quality videos on youtube, and its just him talking over a screen share. Fields medalist Richard Borcherds likewise has posted a ton of lectures of him just talking while he writes on paper.

In fact, I'd expect the highest quality videos to have a relatively low viewership. Most people seem to want Mr Beast or whatever.

[0] https://youtube.com/watch?v=WTEZjR5aNjw&list=PLoaXcYRr65txn8...