Most lenses that autofocus are all about reaching that focus point as fast as possible in order to take a still photo. When you are focusing for video, you're often "pulling focus" (i.e. adjusting the depth where you're focusing) very slowly, and sometimes back and forth between two or more set distances [1]. No autofocus lens is going to know what the director or cinematographer had in mind for the shot. At best you can have AI which will follow a moving object, or look for eyes, but that doesn't solve the general case at all.

[1] https://en.wikipedia.org/wiki/Focus_puller