IMHO they were/are trying to include flying vehicles in the game, but it's the first time they try to do something like that. They went from no vehicles at all in the first 2 games they made to a very buggy horse in their first open-world game (not really, 4 big areas) and then a very open-world game with traffic and driving cars/bike. Not only they need to write new scripts and physics, but with higher speed of movement streaming of assets becomes more difficult (remember pop in in all GTA games and how graphics automatially got worse when driving in watch dogs, with pedestrian's number being cut off every time you got in a car).
Flying vehicles are even worse to do, so for sure we'll have scripted events because are easy to control, maybe they'll manage (managed?) to do flying cars right and we'll have them in the game, but they don't want to give us false hopes. And that's a good thing, overhype always leads to shitstorms. Personally I'd rather have no flying cars at all than shitty ones.
And something tells me this posts will be moved somewhere else as usual (sorry, mods).