I think it makes sense for Youtube at least because most of the times when people click to visit, they're there to watch the video. (I guess you could technically be there to only read the comments and not watch the video but that's very rare)

On the other hand, on sites like Udemy it makes less sense since there are other contents people want to interact with on the page. I think they're just employing a bad ux.

And once we get into spam territory like some shitty sites where most people visit expecting textual content and they just throw an autoplay video at you (which in many cases follow you around even if you scroll away), that's completely different from above two. These guys do this to make ad money.