Because wrestlers are generally selfish people even when they're "good guys"? I mean, obviouslyfaces never come to the aid of other faces because it would eliminate the heel's ability to seem even more heelish: the face is cut off and alone. Having the faces actually band together would destroy that illusion of the heel's power.
It's stupid and makes no sense from a real-world perspective. But you can't have faces get involved in each other's business a lot of the time because otherwise it would undermine the bad guys if it happened all the time. It's good to keep that sort of thing rare, such as when the Usos saved the Rhodes Brothers on SmackDown last week.