One of the many wrong lessons the DCEU learned from the MCU: Other than Hulk and Spiderman, pre-2008, nobody really cared about individual Avengers characters. So if you're making Iron Man, Thor, Captain America, etc., you better tease a team-up along the way. If you're working with the Super Friends, there's no reason to do this. A movie focused on an individual superhero loses impact when we are reminded that they live in a world full of superheroes.