1. Despite having worked together successfully once, they're not exactly texting one another on the regular. That's sort of the "cop-out" answer.

2. The actual answer: in comics, when bad guys attack, heroes don't always join together to help each other. They're off in their solo series, solving their own shit, handling their own threats. Same thing here: since every movie can't be an Avengers movie, they split the threatening situations into different parts of the country and let each hero handle their biz.

In an ideal world, they at least mention, "oh Hawkeye is off doing this thing, so he can't be here," but they can't put him in to further clutter a film that already has to give arcs to a shitload of characters.