If you only saw old WWII movies set in just one theater of war, you would not see all the vehicles used throughout the entire conflict. It’s like asking why we didn’t see submarines in Patton.
Yeah, that’s what we’re seeing in shows like Rebels: Different Rebel factions bring different ships (the Rebels crew has A-Wings and a B-Wing) and then they all come together at the time of Jedi for the big fight.
It’s a cool concept, and even though the Rebel cell we see in Rogue One is the Yavin IV cell we saw in Star Wars, when we see them in the film there’s not really any need to drop troops on the Death Star, so it’s not a stretch to imagine they were moved out of the hanger before the pilots strapped into their X- and Y-Wings.