^^ What he said.
And it's not like they haven't tried. They have, with the JL: Mortal back in 2007 (long before Avengers or Iron man)....the project was deserted for a couple of reasons, including the writers strike (which I doubt was WB's fault).
Oh, and yes, they were planning to convert that into an actual franchise with sequels (solo movies and such). You can look it up if you want.
Circumstances forced WB to abandon that project...I can only imagine what would have happened if the movie was made and released (perhaps, in another universe, it did!).
Superman v. Batman isn't a rubbish title (okay, that's subjective). But, from a marketing standpoint, it's a pure gold. Batman alone can guarantee a big box office draw (Superman could do too, but that didn't quite work out for MOS. It made money, but not enough for a Superman movie).