I'm not sure about The Sims 3 not being customizeable. All I've heard about that is fans speculating about it.
EA didn't intend for The Sims 2 to be costomizeable, either. The only official customization built into The Sims 2 is recoloring the clothing and makeup. Nevertheless, the fans have been able to get into just about everything.
Beyond that, I don't know of any computer game that hasn't been hacked by the fans. They would have to really work hard to keep us out of The Sims 3.