I feel like WB can get away with that more because their characters are more well-known to the GA. I mean yeah the GA probably doesn't know a whole lot of specifics about Flash, WW, etc. But they're at least more recognizable than say Iron Man, Thor, GOTG, etc. The GA probably at least knows the basics of them (Flash is super-fast, WW's an Amazon, Aquaman's rules Atlantis, etc). So you don't need to spend as much time building-them up for JL. They show up in JL, and the GA knows who they are already, and then you can flesh them out in their individual solo movies.