Roddick was never the fighter that Hewitt is. Neither is nadal for that matter. Sure, he competes like hell in a match but I doubt he loves the game or will miss it the way some players seem to need it.
Money is not a factor for most of these guys. It is for some who will come back for 'farewell tours'. It's about doing something that makes you get up in the morning. The training is surely much more grueling and mentally taxing. Compared to that, actual match play is probably fun.