The NFL lifespan is far from misleading. Not sure if you have ever played football but getting tackled by 240 lb. players hurts.
Starting running back runs 20-25 times a game, right?
20-25 X 16 = Circa 350 collisions a season
350 X 3.5 (average playing career of a RB/NFL Player) = 1225 Collisions
1225 does not include getting tackled in practice, preseason games, playoff games.
After a while, your body starts to break down. And that math is just after 3 and 1/2 seasons!!!
The most elusive ones are the ones that play for 8-12 years. (B Sanders, W Payton, M Faulk, E Smith) And those players are very rare.
Third down backs like Mike Alstott and Kevin Faulk stuck around so long because they weren't taking as many carries.