i haven't read them all, but for instance, War Machine was in China battling the 10 Rings terrorist group when the alien attack happened in New York City. in the tie-in comic they actually show Iron Man calling him at the beginning of the battle and War Machine is like, "dude i'm on the other side of the world, i'll get there as soon as i can". he actually shows up after the battle is over when they're at the shawarma restaurant.

so they try to somewhat explain those kinds of situations, but also like it was said by someone else ... in the comics they don't always team up. they are only gong to get together when it's more than one of them can handle. having said that and then just recently watched the new Cap movie i wouldn't be surprised to see some team up movies in the future where you'll see at least 2 of them getting together to fight some threat.