That's not necessarily true. They mention plenty of real-world place names in the GTA games. My theory is that GTA's universe is one in which the cities and such that we know simply declined or never developed, and other places developed instead. So there may be a New York in GTA, but it's only a small town, whereas Liberty became a state. Remember that RDR's towns and areas aren't real-world places either. This doesn't mean the universes are connected, but the converse is also true.
The real problem is that there simply aren't enough resources for Rockstar to fully create an entire country with all cities in a single game, and thus they are forced to reinvent things in an asymmetrical manner each time a new iteration of game systems are released. Otherwise, this Los Santos would be totally disconnected from GTA IV's Los Santos, based on the fact that the in-game TV showed San Andreas' version of LS, which we all know was simply due to the fact that, at the time, Rockstar hadn't designed the GTA V version of LS, yet.