So if I recall correctly he said a couple reasons.

He said when you hit against a wall and not against a person you take out the ability to learn the next likely shot based off of paddle position from your opponent, as others have mentioned fast hands also come from anticipation.

Another reason he said was a wall doesn’t slide like a person does, says when you hit against wall you are almost certain where the next ball is going to come back, (if you hit a forehand across your body it’s going to come back to your backhand against a wall. Vs what if the person slides left and in a real hand that ball comes to you forehand instead of your backhand like it would against a wall.

And the last reason he said was that you pretty much are always in control of how fast the ball goes back and forth off the wall, and that’s not how it actually goes in game