Because what happens when you brake in a modern vehicle is that your ABS system modulates the braking pressure and “pumps the brakes” to slow you down evenly and without allowing the brakes the lock up. The parking brake (the manual cable actuated ones) do not do this which causes the brakes the lock up. That’s just simple physics. The electronic parking brake will not (in any car I have ever driven) stop you if you are moving. It will keep you from moving if you are already stopped but if your vehicle rolls (and say you’ve lost brake pressure because you master cylinder exploded (I dunno, don’t ask me)) your electronic parking brake will not stop you.