Because that’s not a smile, he’s just showing his teeth. His eyes are dead and joyless… definitely off-putting.

Some people just don’t know how to smile in front of a camera.

Or it’s part of their clickbait thumbnail strategy, like how other Youtubers make an obnoxious face because those thumbnails get clicks.

It’s an expression that’s optimized to maximize the click through rate of a video. Unfortunately it also looks hideous

At least part of the problem is the stache.