I can't get over how stupid and inconsistent people at Google/YouTube can be.
They took the time to make Webp a thing. They promoted it. They made it the default format for YouTube's thumbnails. Even the animated ones. Probably saving a good amount of bandwidth.

But... user's avatars, that are so small that you're definitely not gonna see a quality loss from higher compression, are still being served as jpeg.

Why? No clue. It surely wouldn't take much processing power to transcode them. It wouldn't take effort to change code to display them. I can understand that it wouldn't offer that much gains in freed up bandwidth, but surely someone at YouTube would be autistic enough to be bothered that they're using multiple image formats on the same page for no good reason?