It's not just the save file. For one, it'd have to ship with the disc. For two, you'd have to rewrite the logic of how a cpu manages rosters (though this may not be a bad idea either). You also then have to massively increase the number of incoming recruits. You'd have to add the logic for only being able to dress so many players per game, and have to have a way to manage that, because asking the game to keep track of 30 more players per game is a lot to ask. Adam has said one here before it took a lot for them to get up to 70 players. It might be easier for them to add FCS schools, and I don't see that happening either.
All in all, it's not as simple as just adding the players. That's the problem sometimes on here, people think it should be simple to add things without considering all the other things that would have to be done for it to work.
I'm not saying it's unimportant. I think it'd be fantastic. I'd love to have the more realistic recruiting, redshirting, and overall roster management this would provide. I'm just saying expecting it any time soon isn't realistic.
That's a big part of it.