Several of my friends & my mother had hip replacement op.  All fit and active beforehand; after the op during the first few weeks they had to walk with crutches or walking frame certain distance every day.  It was very painful & took lot of effort and determination to do that.  If somebody does not try  before why would anybody think they will make the effort after the op?  Together with all what was said above re- extra danger and difficulty to operate on somebody so large, it makes perfect sense for doctor to refuse.  People cannot expect all their problems to be fixed by somebody else; it just does not work that way.