After the events of Thanos, crime seemed... to disappear. Everyone realized how worthy life was after the disappearance of people, and we're happy where they were. There were of course still a bit of crime, but it wasn't anything the police couldn't handle. The superheroes could finally settle down, have families, and have a normal life. And they did.
Many years later, it was peaceful. Humanity showed Thanos that it could be peaceful without the mass murder- that there was truly life. Or maybe, that's what they were convincing themselves.
A young girl, who was a follower of Stark, decided to bring the avengers, guardians- EVERYONE back together. Because it wasn't perfect. Crime was rising. New threats appeared. It was time.
Avengers, assemble.