1. Topspin can't be hit with a blocking motion. When you're on top of the net, there is no time for a full swing.
2. To generate topspin, you have to have separate grips for the forehand and backhand. There isn't enough time to change grips at the net.
3. When you're closer to the net, you have more margin for error (think court geometry). You don't need topspin to keep the ball in the court.