Because there's no reason that a guy with a 12 blocking rating should ever be on the line. None of the actual linemen are rated that low. None of the rookie classes have linemen rated that low. It's not a situation that should happen. What reason would someone ever have to stick a guy rated that low on the line unless they were trying to cheat? "I can't beat this defensive Madden Moment, so I'm gonna make the other team *really* bad first".
Oh, and yes, call me a fanboy. I love that. I've never played anything other than Madden... *cough*