Originally Posted by kaijunexus
Sometimes a police officer can handle a situation by themselves. Sometimes they need to call in for backup. Sometimes the police department needs to call in a SWAT team or even the FBI.
There's different levels of threats, and each can be handled by varying degrees of response.
People also need to realize that the actors are contracted for a certain number of movies, and Marvel cannot allow them to appear on other people's movies and thus "used up" the movies they're contracted with. And an Avengers movie is an extremely expensive one, an event film that is more suited to show up every few years or so. If the threat isn't as big as an alien invasion like in TA, there's no reason for every Avenger to show up, either.