Because (a) the legs are much harder to hit and (b) a hit to the legs is much less likely to incapacitate an attacker.
The whole premise of them shooting is that there's an imminent threat. If they're shooting, it's supposed to be because they MUST incapacitate the attacker.
From what I've read, not only are they supposed to aim at the center of the target, but they're also not supposed to shoot just once. A single gunshot will kill someone, but not instantly, and in the time it takes for that wound to incapacitate them, they might go on to kill someone else.
Which when you think about it is the worst of both worlds: the police kill someone, and that person still manages to kill someone else.
A little later.
I was curious enough about this to make sure I wasn't just making this up (power of false memories and all that). Some arguments against "shooting to wound":
* It supposedly takes ~250ms to fire a round under optimal conditions, and just ~150ms to rotate/translate a limb from one side of the body to another; the target moves faster than the officer can shoot.
* Increased likelihood of missing the target also implies an increased likelihood of hitting something that isn't a target (again: something I would agree militates for officers not shooting at all, but still).
* The places on limbs where you're most likely to hit a target are going to cause them to bleed out anyways --- just not in a fashion that immediately incapacitates them. The bulk of the mass of an arm or leg is also where you're most likely to hit a huge artery.
* You shoot the legs of an armed attacker and you've left their arms and hands functional. Which means that the option (which doesn't really exist but stipulate that it does) to fire at the legs is situational; maybe it's something you could do with an attacker some distance away from the victim and carrying a hammer, but it would be totally ineffective against a target with a gun. You want rules that work in all situations.
* Assuming optimal conditions in a police shooting incident is silly, because everyone involved in that situation is completely and unavoidably overloaded with adrenaline. Police are going to fire multiple times, even if the guidelines say otherwise. They are going to turn in their worst firearm performances, not their range performances, because they're impaired. Stipulating that, you want them picking the part of the target where they're most likely to hit what they're aiming at.
There are other less credible arguments against "shoot to wound". For instance: police are equipped with rounds designed to penetrate but then mostly be stopped by the mass of someone's chest and surrounding clothing, and those rounds won't be adequately impeded by an arm or a leg. Ok, so change the ammunition.