How do injuries never heal? Some do and others never do, to the point that you are never as good as you once were before the injury(ies).
How do you stay fatigued if you are able to rest and recover? Over time with the punishment that an NRL player has to endure, chronic fatigue can and does become an issue for a lot of players as father time and all of the above start to add up. That is why it is so, so hard for players to have a 10+ year career in the NRL playing at their best year in year out and why so many of them pull the pin in their late 20's, early thirties and head of to England or retire. I'll grant you that with modern practices the sports boffins have managed to prolong the careers of many player way past what was once possible and why we are seeing so many 300+ game players recently, but they of course are still the exception.