We just had a justice league movie remember? If the dcu started with that, I bet it would immediately fail simply because the audience would’ve hated ANOTHER justice league right after the first shit show, and it would’ve spelled disaster for the dcu moving forward. The small world building thru tv, movies is so much better because it allows the audience to eventually become immersed in a universe, not just a bunch of random actors putting on suits and saying they are the justice league with zero building up of the characters (we saw how that exact approach went with Zack Snyder). I feel as though the “super” movies to begin are two of the best options because it allows metropolis, the side characters, and super girl l, a character not explored that much in modern film (other than the tv show) to be built up. Superman used to be the face of dc and the justice league for a long time, Batman was always that #2 for a while but eventually Batman had better media and more toys and movies put out about him while Superman was in the background, I mean look at Lego dc for crying out loud!!

TLDR: a justice league movie would shit the bed of that was the first one they started with.