Human beings desire interaction with other human beings.
Most people hate dealing with ACD phone systems that route calls. Everyone presses #0 hoping to get a live person on the phone as soon as possible.
Now if people feel that way about everyday transactions they're going to definitely want to deal with a human being when it comes to something as important as education.
Also if computers replaced teachers that would be the end of essay questions where students explain their reasoning or use conceptual thinking. It's the difference between knowing the "right answer" and (knowing why) it's the "right answer."
Every test would have to be multiple choice or true/false. Computers also would lack the ability to adjust or explain a concept to an individual student in a way that they comprehend.
It would be even more of a "one size fits all" learning model than what we have today.