It depends why you're playing the game. Someone like me, or say the aforementioned Brett Favre, who just wants to go out and beat people in a competition, are gonna wanna play as long as possible. But it's a perfectly reasonable, human thing to go "hey man I can play this game for a couple years and bank a few million" and then just walk away and go do whatever other things you truly love. It doesn't even have to be about the risk of long-term injury. Just because you're good at playing football doesn't mean it's your greatest passion in life. Most people don't have jobs that perfectly align with their personal interests, they do it to make money so they can continue to function in modern society. Getting a job that you can do for 1-3 years and become part of the 1% (or, well, maybe 2 or 3% I dunno the numbers okay) is a pretty damn good deal in my book, most people have to work 20+ years and never even reach millionaire status.
Anyway, lolniners. And holy hell Reggie Bush is 30 already? Feels so long ago...
next-day edit: Apparently Borland "only" made $500k off the first year of his rookie contract, so it's not like he's totally filthy rich off it, but that's still enough to retire to a modest lifestyle and supposedly his family comes from money anyway so he doesn't need it much.