Because that’s action movie nonsense. The issue is that they pull out their guns too easily, not the place they aim them. Guns are not a tool for disarming or wounding. For one thing, a leg shot can easily kill. Second, it’s extremely difficult to make that shot with a pistol, even for trained police. Third, if someone has a weapon and they are about to kill someone (which is the only time police should be pulling out their guns), a leg shot might not stop them quickly enough. And disarming someone with a bullet is absurd except in very rare stand-off situations where the person is sitting still, and a sniper has had time to get set up.