Because regen approaches zero as you stop. To get a clean stop it has to use the breaks.
Regen torque or regen power? With what type of motor?
Power for regen and acceleration inherently goes to zero at a stop, because the definition of power requires motion.
Torque is a different matter, and the permanent magnet, phase locked motors are fully capable of delivering regen torque all the way to zero rpm.
The induction motors with their slip speed ratios can’t, which is presumably why older Ss and Xs still don’t have Hold mode as an option- they would have to blend the brakes, and Tesla hasn’t chosen to do that.