no one would. you'd spend your time traveling around, trying to hunt, find food and other people, and disposable energy sources. you would get plenty of exercise through one way or another, but trying to maintain a hardcore bodybuilding lifestyle would be a lot of wasted time. plus if you for some reason got a serious injury like torn rotator or pec, then you = fucked.
but if you're claiming that somehow the world continues to operate as usual but just with the people gone, then yes, why not. i mean after all, isnt not having a real job or responsibilities what bodybuilding's all about?