2. Not enough pay out. Sure they are in better shape, but they weren't an active person before so their new fitness is going to go unused anyway.
3. The reward cycle isn't fast enough. I know some people claim working out makes them feel great(endorphins and all that), but the rest of us just get tired.
4. Most workouts are just absolutely boring, lift this heavy thing 20 times, run in a circle till you drop.
So the ideal workout would be mentally engaging and provide feedback quickly. Off the top of my head blacksmith or rock climbing come to mind. Some of this is already being serviced by adventure runs(mud runs, zombie runs) etc.