It is pretty obvious as a creator. The analytics available encourage experimentation. You find that mentioning subscribing or making a more visually engaging thumbnail will significantly boost important metrics like CTR. CTR is a metric almost entirely dependent on thumbnail image, although title and description play small roles, and CTR is an incredibly important metric for getting the content served by the algorithm.

And it isn't entirely true that including a face mugging for the camera is always the most visually engaging (although representations of people are very attention grabbing). A person in the image is a character for a story, the whole "worth a thousand words" is true in that a whole narrative can be compressed into a single image and viewed with a glance. A person or other sentient being is a character for that story (I'm sure cat thumbnails do well too).

Just try to think of an interesting narrative involving exclusively inanimate objects. Kinda hard; the whole "animate" thing seems to be necessary to give a before, middle, and after to events (ok, maybe collisions, explosives, and rockets might work, they'd probably do well as thumbnails too). A thumb with Linus looking surprised or disappointed or puzzled gives a short and incomplete narrative about the object he is looking at and how it made him feel those emotions. Part of this is that watching the video will give you a more complete narrative.