You have to consider the technical limitations as well. It was designed to play on the PS3 and Xbox 360. I imagine the next generation GTA will offer more of what you are looking for.
If the game randomly JIT generates all local phenomena while keeping just enough coherent higher level data to suspend your disbelief, that's only the storage size of the high level information.
However, when a player begins to specifically perturb that generated phenomena at the detail level, how do you prevent leaving an ever-storage-requirement-increasing trail back through the player's space-timeline? Reconvergence to normal over a reasonable timespan?
I assume this is a thought about problem, so links greatly appreciated!
[1] https://en.wikipedia.org/wiki/Mercenaries_2:_World_in_Flames