Why would he bother writing his own physics engine if there are free and easy solutions already? People who invent their bicycles don't end up shipping stuff in

Why would he bother writing his own physics engine if there are free and easy solutions already? People who invent their bicycles don't end up shipping stuff in two weeks with no prior programming experience. http://cocos2d-iphone.org http://cocos2d-iphone.org (which uses:) * http://code.google.com/p/chipmunk-physics/ http://code.google.com/p/chipmunk-physics/ * http://www.box2d.org/ http://www.box2d.org/