The very fact that it's history, and we're looking back with knowledge of how things turned out and a fuller picture of the different parties and their intentions, leaves elusive that contemporaneous confusion that often describes "why" for participants much better.
For me, it's the sheer chaos of being alive. There's no sympathy in McCarthy's universe, no room for sentimentality. It's bleak as hell. If there's loving, human contact, it's fleeting, and erodes, usually quickly, like the unforgiving landscape of all his books. There's slight redemptive quality just enduring life, enduring existence if you have the strength. He's one of the best American existentialists. "I'm still alive", is both banal and heroic in McCarthy's world.