Why can't they make breast implants out of the fat a person already has?

I’ve been wondering this for a while, and I just asked Tim what he thought. He said “my girlfriend has ample boobs, I don’t know, and I kind of don’t care”. So, just to avoid a lot of wasted energy, everyone, I DON’T WANT (or need) FAKE BOOBS—you don’t have to talk me out of getting them. it’s okay! ;-) From a scientific point of view, what’s to prevent them from sucking the fat out of your thighs and sticking it in your breasts? They already take fat out of people’s butts and put it in their faces. What’s the difference?