Kobe Asks: I heard that you can’t fly in Liberty City anymore. What is up with that?! Flying is one of my favorite parts of this series!
This is incorrect. There are no airplanes in the game, but flying is still there. One means of this is the helicopter. As Dan pointed out, this is just one city and a plane really doesn’t fit into that very well for gameplay. Flying is still very much a large part of the game.
I don't post much in this section, but I don't think it has to do directly with 9/11. It has to do with the fact that it doesn't fit with gameplay. Also, you say past titles had one city but had planes. It's because the HD era is supposed to be more realistic than the 3D era so this is why Rockstar took this approach with GTA IV.