Wouldn't a freshman or sophomore starting cause them to be more vulnerable to career ending injuries? Especially the linemen. Think M Dudek...
This is merely one anecdote, but it speaks to what you're saying. At Penn State, James Franklin has sold RBs on getting minimal reps that first year, getting more reps in year 2, and then starting by year 3, graduating early, and going pro. Saquon Barkley got thrust into the starting role his freshman year, but that's certainly not the preference. The Miles Sanders example is the model he's looking for -- learn behind some great backs the first few years, have one breakout season as a starter, and go pro early with minimal physical damage.
Miles was the #2 back taken in the NFL draft, and many say it was partly because he only had ~300 or so career touches. He's still fresh.
That model is really attractive to elite level RB recruits, particularly since RBs are likely to cut their pro careers short by getting 800+ carries in college. Ideally, you keep the touches low early, have a big year or two, then bolt while you're still fresh.
Now, that doesn't necessarily apply the same way to linemen and defensive backs and whatnot. But still......there's something to be said for keeping the physical toll light early on until it's time to really shine and make a case for the NFL. Getting thrown out there as a freshman when your counterparts are a few years older isn't ideal.