That's because nobody could have predicted how badly DC was going to fuck up their movie franchise.
It's bizarre. Just do the same thing Marvel did. Intro movie for each hero, then Justice League.
Not intro movie for one, then throw a bunch into the next one, then a movie for one you threw into the last one, then Justice League adding more heroes we haven't seen yet followed by their own movies later
Who the fuck is in charge there
Technically they did an intro for Wonder Woman too. She got her own solo movie before JL and it did pretty well.