They don't. As you may know, the CW can't use Batman, Superman or Wonder Woman on the CW verse. The WB film division forbids it. Additionally, they can't use Ted Kord, The Blue Beetle, because the film division has plans for him. Perhaps anyone with plans can't be used by CW.
So if Superman, Batman and WW exist, they can never be seen or used. This means no matter how much the world is threatened by the events of Flash or Arrow, Superman will never show up to help. That means Superman will never be Superman. Perhaps Krypton never blew up and Kal-El is just kicking it with his dad doing science. This means no matter how scary and effective Batman is, no one will ever hear of Batman, and no stories will ever circulate about the Batman of Gotham City. This means Batman will never be Batman. Perhaps the Waynes were never shot. And Wonder Woman will never help either, and the news of this Goddess-Warrior-Diplomat will never be overheard on TV in the CW verse. This means Wonder Woman isn't Wonder Woman, and never will be. Perhaps she never left Themyscira. Perhaps Themyscira was never enchanted by the gods. I think it would be really cool if these things were alluded to, to cement that our heroes are the heroes of the Universe, and not someone we haven't seen yet.
Here's why that's awesome:
1. This means Arrow is the most awesome vigilante in the CW-verse. There's no one scarier, cooler, better at detective work, with more gadgets, more money or anything like that. There's no one else who can stop Ra's Al Ghul. Arrow is as good as it gets. The producers have said as much when asked about Batman showing up, they like Arrow being the best there is at what he does, and aren't interested in bringing in someone to out-Arrow Arrow on Arrow.
2. This means that Flash is the goodest good guy, and the most powerful superhero in the CW-verse. There is no one to look up to, no one who inspires people more than the Flash. There is no Superman that Flash is trying to be like. Everyone, every superhero with powers, looks to Barry Allen, the everyman, the nerd with girl problems, the down to earth good guy, as the epitome of what a superhero should be. Barry deserves that honor, imho. He's a great everyman character.
3. They don't have to coordinate with the film division or be controlled or limited in any way. They can just tell awesome Flash and Arrow stories and let the films fall where they may.