It's just less accurate, more error prone, and more limited. Especially in bad conditions and when there is low contrast.
Stereo vision can also be fooled by things like reflections.
Stereo vision at the human level also requires head movements and so on.
Also people don't realize how important blinking is. Any camera system that is not thoroughly self cleaning is going to have significant problems after a relatively short period of time. We don't have cameras built to withstand the elements day-after-day like our eyeballs.