As a heavy YouTube user this simply is not my experience. Not that it never happens, but it’s not the default to the point that you can just assume that. In most cases your assumption is wrong.

Maybe it just depends on your viewing habits and what the algorithm pushes. I see loads of top 10 movie clips (e.g. sci fi movies on Netflix) in my feed that have an AI generated thumbnail and when I’ve viewed some of them there’s been no apparent AI use in the video.

I am also seeing AI generated clips where the thumbnail doesn’t look AI generated. That one’s very annoying as I don’t want to watch AI generated videos at all.