Didn't Tony have a line about needing "to do this on my own" in IM3?
When we went to see TWS, during a scene when Cap and Falcon were in battle together, my friend leaned over and whispered "this should be Iron Man helping him out!" Of course, we've since had people complaining that Iron Man "took over" Civil War and looks set to do the same in Spider-man: Homecoming.
I'm in the camp that says issues like Diana's absence during the battle for Metropolis are a peril of shared universes and we just have to accept them like we do in comic books (or like when we didn't see the ENT-E during DS9's Dominion War).
Like I said, the compressed timelines of stories like IM3 and WS mean it's believable that the other characters wouldn't have enough time to show up and help out.