- They’re not sewn at the hip.
- They still don’t like each other that much.
- Some things are just not anyone else’s business.
Probably having a life or doing things on their own while something is happening to the others?
Like Thor is in another realm while Iron Man 3 is happening in California/China (and the places Iron Man 3 takes place in) meanwhile Steve and Natasha along with Sam and Sharon are dealing with Bucky and other bad guys, while Clint is in a top secret mission for SHIELD and Bruce is walking around in an unknown region not wanting to be found.
Also they only assemble when there’s a super global threat that requires them to get together.
Unless its EMH, then avengers assemble as often as the roster changes, and that changes more often the Clint probably...
Probably having a life or doing things on their own while something is happening to the others? Like Thor is in another...