And though it would certainly have been cool if the live-action DC Universe had been one single reality, where Gotham really is the prequel to everything, and the Flash and the Arrow could show up in Justice League movies, I think Marvel's Agents of SHIELD probably showed us why this wasn't such a great idea. Before SHIELD fans start stamping their feet and protesting that the show's gotten better, or whatever, let me first say I'm not talking about quality here. The real problem was that the show started out making choices based on what was best for the films and not the TV series. It kept itself from introducing characters that were film-worthy, and worst of all, had to wait for Captain America: The Winter Soldier to come out and for people to digest it before heading into certain directions. The reverse could have happened to (and yet may), where the show's going well, but everything must be interrupted or sent into another direction by movie events.
So imagine a TV DCU that's a slave to the movies (because of course it would be, and not vice-versa), where the Flash can't meet Firestorm because they've suddenly decided to keep him for the bigger budget film, so that subplot goes nowhere... Or indeed, where those heroes who are currently badass solo stars would end up in a subservient role to the big guns (which will still happen, but they won't have years of badassery under their belts). No, better to keep things separate and autonomous. (Hey, how about making the movie guy Wally West to change things up?)