[blackbrokkoli]

Because otherwise people would just put a 1-frame endcard in their video with whatever they want to have as a thumbnail?

And if the thumbnail is picked randomly, you are just rewarding videos that show dramatic content with text overlay at all times. And the best-researched Tom Scott video might get randomly punished because the thumbnail is a brown-grey blur in the middle of a transition.

The existence of the medium "thumbnail" is not more of a problem than the existence of book cover. Breeding a healthy culture of what should be on it is the problem we should focus on, both by audience behavior and suggestion algorithm.