Because they aren't 7 feet tall swatting away passes like Shaq blocking shots.
Browning has been playing more meaningful snaps. Borland is playing a LOT less than last year and there's a reason for that. Browning plays more meaningful snaps in critical situations
If Ohio State wants to win a national title, Borland can't be getting most of the snaps. That's the general consensus on this site and the coaches apparently have that feeling too.
So no, it's not a mental issue, it's an observation that's been supported by his performance and snap counts.
Most of rhe B1G isn't running I formation like they were 10 years ago. Northwestern, Minnesota, Illinois, and Nebraska got away from that. Look at how many spread out and run the majority of their snaps from the shotgun and run with the QB. Iowa, Wisconsin, and Sparty are about the only teams that don't play this way. Even Sparty is running option plays and getting their QB to run. There are way more athletic guys lining up at skill positions in the B1G.
I'd argue most teams want to play in space rather than in the box and that's where Borland is a liability. Sparty wanted to play in space with quick routes and free up their receivers to play in space. And that's the shift. Maybe teams look bland doing it and some still play smashmouth football, (hello Iowa and Wisconsin), most of the conference doesn't. Maybe Minnesota and Northwestern don't have the best athletes, but they force teams to play in space and make open field tackles. That's the shift and maybe it seems bland and not as dynamic as our offense, but we still have plan and prepare for that each week.
Browning has a nose for the football too. I'm skeptical of Borland coming off the edge in the 4th quarter and sacking Lewerke. Borland is a step or two slower and that takes away from the defensive scheme of playing fast, keeping things in front of you, and swarming to the ball.
But Borland is a backup at this point to Browning. I understand what you are saying about Iowa and Wisconsin backers, but those teams typically get exposed against speedy, passing teams, and that's the difference on defense from winning B1G championships to being a 9 or 10 win team
Borland would be an all big ten talent at Iowa, Wisconsin, etc like 10 years ago but the game has evolved where his skillset is exposed.
If he played college football 10ish years ago in the B1G, he would be having 120 tackle seasons against teams running the I formation. Nothing against Borland, but that's his strong suit and unfortunately, the game has shifted from that.
It's hard to justify him being on the field, especially in passing situations against fast teams. He can't be covering anyone with speed.
Perhaps intangibles since he is a captain. Borland can tackle and that's one of top skillsets. He can play in the box but he's only a 2 down linebacker in college.
To answer the latter part of the question, Browning is going to continue to play more. He's outplayed Borland
Why would you leave Ohio State for the NFL?
Money, ego, and less demanding schedule. With recruiting and other things, there isn't as much of an off season as there is in the NFL.