I think superhero team ups are a completely different beast. In most ensemble films, the characters were created to be an ensemble. They exist in the same context, within the same narrative, within the same genre, and are each other's supporting casts.
With The Avengers, and by extension the Justice League, you have an ensemble cast made up of characters that were designed to be heroic leads in their own largely unrelated stories that fall into the conventions of largely unrelated genres. Each one of them has ten times as much baggage as any individual character in most ensemble movies, with their own arcs, their own supporting casts, their own themes, their own villains, and their own genre conventions that have very little to do with each other. It may seem natural to us that they're all on a team together, because we're used to it as fans of a comic book franchise that has been growing for decades, but if you're first ever exposure to these characters that on the surface have absolutely nothing to do with each other and belong to completely different genres is them hanging out like it ain't no thing, that's not going to work for most folks without spending a lot of time establishing and growing these characters and the world they exist in.
It's simply too much stuff to get out of the way in just one movie.
.