The career length for a running back in the NFL is 2.57 years.
If I were a betting man, I'd wager that number is even lower for RB's who stayed for a 5th year. I'm guessing that NFL teams tend to take a dim view on running backs who have high mileage from their college years.
There will always be some that buck the trend. For example, Ricky Watters was in the NFL for 11 years (missed his rookie season due to a broken foot), but for every one of him, there are at least 50 who didn't even make it to the "average" lifespan.
If a running back can get a favorable draft projection, then it makes perfect sense to leave right away.