Why is there no more longevity with today’s RB’s?

durability of most running backs was always below the expected longevity of other positions. I think I remember seeing the "average" was something like 5 years and that was maybe 2 decades ago. But the all time greats either ran on exclusively grass or mostly grass. Grass hurts less to fall on, and defenses do not get as much speed before they hit you. Couple that with overall greater speed and training for defenses and you have most of the culprits.