Who is to say that some of them don't? I don't think that is the way it works though. Momo was able to test out her new body by having it connected to her old body via cable, but that's different than copying her mind over. Chassis are not cheap either, and job opportunities for AIs may not be that great yet, so those able to afford multiple bodies seem like a limited group. And finally there is the point that having multiple bodies is problematic from a personal and a legal aspect. People can get mixed up and not recognize others if they've changed their hairstyle. Imagine if you could change everything about yourself. Different color, shape, size, gender everything... all on a whim? And imagine an AI criminal. A robot robs a bank, steals a bunch of money. Leaves no DNA or fingerprints obviously, so people get a physical description. To bad that body's already been discarded three blocks away and Pintsize is toddling back to the apartment to give a vague and sexually explicit comment about how he spends his time when he's out...