Not to mention that being in shape is one of the single best predictors of how well someone's going to heal up from a major injury. A fat person isn't going to be keeping up with physical therapy if they couldn't be assed to stay in shape in the first place. Not to mention that it's often imperative to build strength back up in injured body parts if you're going to ever regain full function again.  It's just not really possible if someone's starting out in a situation where a huge amount of dead weight is being ungracefully distributed all over someone. And of course there's just the desire to be out and about in the first place. People who live on their couch don't have the motivation to push themselves to be out in the world again. And healing up from major injuries the right way is often a rather painful process that you NEED to want to push through in order to do so.