Because it’s not about roster spots. It’s about finding a Miami caliber running back who wants to be 5th string. Is there a four star running back, or even a highly rated 3 a tar running back who will sign at Miami to be 5th string or worse? Of course there isn't. Maybe Alabama or Ohio State can load up on superstar running backs but Miami isn’t exactly on their level recruiting wise. Besides, if the first, second, third and fourth team running back all get hurt, your running game is already done.
As for having a 16th O line vs a 6th running back, I’m taking the lineman all day. You play with 5 linemen at a time. You only play with 1 running back.