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.
As I understand it, police are generally taught specifically to shoot center of mass because aiming there is the most effective way to reliably and quickly end the immediate physical threat. This also ends up pretty close to maximizing the killing effectiveness of shots, but is not the primary focus.
The reason is that you are more likely to hit the target rather than what some propose, that they should just wound him in the kneecap, an almost impossibility.
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.