Police are trained only to shoot to kill because pulling the trigger needs to be a big event. If you think that "shoot to wound" is an option, then of course you're going to pull the trigger more often. You'll become more comfortable shooting people and, presumably, you'll also end up killing people more often.

Other reasons for "shoot to kill" relate to effectiveness: in the energy of the situation if you aim to wound you may either kill by accident, or not hit at all.