Avengers: Infinity War picks up about two years after Civil War, and in the time since everyone has been doing their own thing.
The other members of Cap’s team, Wanda and Sam, have already been seen joining the fight in Infinity War. It’s just the conspicuously absent Ant-Man and Hawkeye who are the question marks.
The events of Civil War likely made him no longer want to be a part of the constant struggle of superhero life.
Hawkeye is one of the few members of the Avengers who has a real family life, and he probably went back to his wife and kids and retired after that film.