Comment by YeGoblynQueenne

There are so many places wheels can't go and legs can. For example, most rough terrain is right out of the question for anyone using a wheelchair. There are all-terrain wheelchairs that can go over some rough terrain but you just can't move across the same range of terrains with wheels that you can with legs. Even Mars rovers have to carefully avoid areas of e.g. deep sand where they can get stuck etc (and they still get stuck occasionally like Spirit†). Hence also all the specialised wheel designs like tank treads.

If it were possible to make legged robots with the same fine motor control and dexterity as human legs, let alone four-legged animals' legs, we'd be way ahead of where we are right now in mobile robotics. We can't.