I don't understand the assumption that lack of prior knowledge of the characters is a dealbreaker. Nobody knew who Luke Skywalker and Princess Leia were when Star Wars came out in 1977. Nobody knew who Marty McFly and Doc Brown were when Back to the Future came out. Thousands of movies over the decades have been built around characters the audience had never previously heard of, because they never existed before their movies came out. So where in the world does this bizarre notion come from that unfamiliar characters can't work in a movie?
The problem is you're dealing with characters who have decades-old history behind them which carries a LOT of weight with them in terms of the general audiences. It's part of the problem the TNG movies had with not wanting to connect too much to the series itself or the ones on the air, and with the newest movie wanting to be pretty much be a reboot.
Superman, Batman, Wonder Woman, Aquaman, etc. are NOT new characters in the same regards as other movie characters because these characters are decades old and general audiences are going to be scared by that. Thinking they'll have too much weight on them from the past that'll leave them lost and confused.
Besides, look at the MCU, all of the characters got "origin movies" before the team movie to establish them. Rush right into a JL movie and people might be afraid that it'll connect too much to Batman movies they've probably not seen, that the characters without movies aren't familiar enough to connect to them (this is a problem I think Hawkeye suffers from in the Avengers. He isn't established.)
Besides, whether or not you "need" origin movies for these characters or not is beside the point. Aquaman and Wonder Woman are strong characters who DESERVE their own movie.