Food, stipends, housing, tutors etc not paid for if you come back.
Even if you never make an NFL roster by staying, I dont get the stance that leaving early to be undrafted is a sound decision
Leave early - No NFL, no degree, nothing gained
Stay one more year - More development = Increased chance for NFL (no guarantee but can't hurt)
College degree from UM
One more year to play football! Most people would kill to have another year of playing ball and living the college life. Especially big-time D-1 ball
There is literally no benefit to leaving early and not making a roster or practice squad. Staying doesn't ensure anything but it has benefits and upside that make it an easy decision when you take emotion out.