For star drive, the force is transferred between the walls of the bit and the walls of the screw flush against each other. Like with gears. This leads to minimal wear and allows for wide tolerances and sloppy angles between bit and screw.

Square bits on the other hand drive the edge of the bit against the wall of the square hole. The edge can cut a circle into the screw, and the square head gets turned into a circle by wear. Thus required tolerances are much lower, and screws don't fall off the bit. If they did fall off you'd know to change the worn bit.