Doesn't matter if the versions of them seen in Batman Vs Superman: Dawn of Justice are new, even the most casual film goer knows Batman like the back of their hand.
And while the others haven't had successful movies of their own yet, they're still been cultural Icons for longer then Marvel as we know it has existed, especially Wonder Woman. The Flash is on his second Live action TV show. And their Animated incarnations are what multiple generations have grown up on, while Marvel animated only ever really hit it off in the 90s.
And people are acting like Marvel has done no introducing characters in others people's films first. They have done plenty, but it's taken till Phase 3 (which is still yet future) for anyone introduced as a side character to get a solo film (and those will be ones introduced in Phase 3). DC isn't going to waste the time they spend introducing future leads in supporting roles.
DC doesn't need Marvel's strategy. The Avengers big three had never had anything good in Live Action before the MCU, only their number 4 player had. So the MCU had to introduce those Characters altogether, not just those incarnations.
Meanwhile Superman, Batman and Wonder Woman have actors who are cultural Icons solely for playing them. Affleck and Gadot only need to differentiate themselves from what came before, and they can do so in only a few scenes each. They don't need to establish the character altogether.
And It'll be the same with The Flash more or less. The rest are in their importance the same as all the marvel characters who will be guest stars before getting their own film.