That's because the Justice League has no business being its own movie until all the characters have been properly introduced to the audience. And, no, it's not about copying The Avengers. It's about making ****ing sense.
Starting with a team movie and spinning each character off into their own films is a terrible idea. Seriously. So dumb. People could watch JL and then go on to watch WW and think "Eh. Nah. She's not as much fun without Batman or GL, etc." And then there's the problem of properly introducing each and every character to the audience. The Avengers had already been preceded by solo films that gave the bulk of the intros for those characters, so the most it had to do was jog the audience's memory. JL would have a much larger problem on it's hands. Sure, the audience knows whats up with Batman and Superman, and I'm sure a few will remember whatever the hell was done in GL, but they'd have to give full intros to Wonder Woman, Flash and whoever else might be in the mix.
Just a bad idea IMO.