Again: Inherently fantastic characters are easier to introduce in an inherently fantastic setting, which is why the GotG comparison doesn't wash.
This isn't that hard a concept to understand. No one asks why or how dragons exist in LotR or GoT, but if a film were to open with a dragon flying through modern-day Times Square, surely you'd want to know what it was doing there. Similarly, characters like Rocket and Groot would have required a hell of a lot more explanation had they been introduced in an earthbound setting.
Then you can explain it within a scene or two. You definitely don't need a 2-hours movie for that.
The Matrix starts with Trinity doing some weird kickass shit and then teleporting via a telephone booth. Then, 30 minutes into the movie, Morpheus explains that the world we see is really a virtual reality feed to us by machines that harvest and treat humans as their energy source. The end. The audience get that and the movie is successful. The whole universe (the basics) and ideology of the Matrix was explained within 5-10 minutes.
Audience isn't stupid, at least not that much. And when you go to the cinema to see a movie that stars Superman - you know, the guy from another planet, who is inhumanly strong, can fly and can shoot laser beams out of his eyes - you have to be prepared for crazy characters and scenes out of this world.
There are a lot of movies, books and games that merge modern-day realistic settings with sci-fi or fantasy ones and they just throw you into the world, in the middle of some action, and explain the universe as you read/watch/play it. Why can't we expect the same from comic book hero movies? Why do we have to take it slow and dedicate solo movies to each character.
And I prefer the Superman vs Batman to establish the universe and characters of TJL and
then have separate movies that focus solely on Wonder Woman, Aquaman, The Flash etc. where those characters are already established as existing in this world and serving as super heroes.
I think you're really underestimating the intelligence of the average cinema goer, who will roll with it. It's people online who seem to think this shit is more complex than 'there's Wonder Woman, she's an Amazon' as an introduction.
Exactly.
"Here's Wonder Woman - she's an Amazon warrior princess."
"Here's Aquaman - he's the Atlantean, ruler of Atlantis and oceans. He can do stuff with water and can talk to fishes."
"Here's Flash - he's a cop and is the fastest man on Earth."