The difference is, Sam and Dean would rather disobey their fates, their orders, to save each other, to save the world. They love one another in a fundamental, necessary way. More than they ever loved anything or anyone else. I just watched the fight scene at the end of When the Levee Breaks, and then Bobby chewing Dean a new one at the beginning of Lucifer Rising.
Absentee father/God, check. Righteous older brother, check. Rebellious younger brother, check. Apocalypse should go off as scheduled, yes?
But then there's Bobby! He's humanity, for the boys. He slaps them upside the head, calls them idjits, and MAKES THEM be good.
So, yeah. Bobby saves the world.
Also, maybe Chuck set all this up so that Michael and Lucifer could have one last chance to reconcile?