They have been in college 4 or 5 years already. They have played this season under significant stress, restrictions & minimal if any contact with family/friends. If the guys have pro potential, it isn't likely to improve much in the next year outside of special cases like Tay Martin. The guys without pro potential are probably ready to pursue a regular job or may even be a bit burned out with football. Most of their lives have revolved around sports. Even with a scholarship, being a broke college student gets old after awhile. I'm not really surprised at all.
As I said about Dillon Stoner in some other post, your time as a starter for a program like OSU's is the time when you build up the social-capital that you can trade in for opportunities after you're done playing that us regular joes don't get (from boosters, maybe, but at the least from fans who are in a position to offer such...it really is
who you know). You want to maximize how much of that value you build up.
I will say this about Dillon, though: he's had a lot of impact/playing time over the last 4 years, so that's a little different than guys who finally made it out onto the field as juniors, and had their first full time starter situation as a senior. For the latter, staying one more year may double (probably more) that social capital.
I also come from the perspective of the many players I've heard say they love the game so much they'd do it for free (whether that's true or not is beside the point). My closest friend had such trouble giving up his playing days after high school that he played college ball at a low level, then semi-pro for a while at a level of financial backing worse than many high school teams, then he coached that team...all because he loved football so much. I hope all these guys aren't just burnt out. That would be, at least a little bit, an indictment of Mike Gundy (for us) and the NCAA (for all teams).
How many college starters do you realistically think consider themselves not to have a decent shot at the NFL? Even at that level, self-belief is typically not in short supply.
For these guys that do think they have NFL potential, it's a tradeoff: risk of injury for the chance to boost your football resume. Some guys may be legit scared they can't repeat the success of their most recent season. For Ogbongbamiga, I agree he's done so well that choosing to avoid further risk of injury is as good of a choice as any.
But
all these guys aren't even close to being as good as they can be, which is why NFL rookies don't usually light up the league. So I guess we just disagree, especially for the borderline NFL guys, that they don't stand to gain much. ...unless you think having your best season yet and getting drafted in the 3rd round rather than going undrafted doesn't amount to much. Sure, Joe Burrow wouldn't have gone undrafted, but he's pretty much the best-case example. From projected 5th rounder to #1 pick in the draft...in the space of one senior season.
Also, consider that most people look back on college as "the good 'ole days". Now imagine what that's like when you're mr. big man on campus (or at least one of them) starter on one of the winningest power 5 football programs of the last decade. And you're on a full ride scholarship. With ridiculous amenities at your disposal.