He's explained it's pretty much for the same reason the OP did it. It's possible to prove it works, especially when you're releasing a video each day. Youtube at this point is about optimizing clicks. People like Mr.Beast generally know their video title and thumbnail before they even start production on the content.

Unconvinced. LTT has 1 or 2 good videos for every 30 videos they release. Maybe the face helps people click the bad videos, but the good videos stand for themselves and it's likely that he would be successful without playing the game. (His all time most popular video doesn't have a thumbnail of him making a weird face. It's his most popular video of all time because his guest was exceedingly entertaining.)

(Also, I don't mean for this to be an anti-Linus post. How he built LMG is a great story and he deserves the success he's achieved.)

Very offtopic but, at first I thought you said L1T and we almost had to take this outside. Level1Techs is awesome. Can't say much for LinusTechTips as it's targeting a different audience, though the young man does seem charismatic.

Unfortunately they do A/B testing on the same video to confirm this.

I can’t stand the dumb faces either. And yet… I catch myself being more likely to click on them when it has a face on it. It’s the weirdest thing.

He's a video explaining how well that kind of clickbaity thumbs works.

Said video: https://www.youtube.com/watch?v=DzRGBAUz5mA