They're easy to parse and feature the thing the video is about and the face of the person in the video. I don't have any particular love for them either but it's easy to see what they are trying to achieve, you see this on every type of YT video imaginable welcome to the internet friend
It's just a way of making the thumbnail more interesting. Human (mostly) face expressions communicate huge amounts of data which makes the thumbnail inherently more distinctive.
Selection processes (which happen in YouTube naturally because of the high traffic) have this outcome.
Some creepy/weird outcomes of these processes are exemplified by the following picture:
I remember when linus first started doing this. He made a video explaining his motives moving forward. They're only making these videos to generate income. Let them do what they need to.