I think it's just artistic choices. There definitely are many elements that make is seem like Los Angeles and the greater metro area. I would think they could if they wanted to, as Ubisoft does for Assassin's Creed. But it is more work to actually make it like it accurate, and probably will leave you more stuck in traffic, as some of these location are like nearly 200 miles 320km apart.
Plus, some people complain when their city either gets left out, or is the location of a crime or crime zone. I saw people complaining about The Valley not really being included. Others about Redondo Beach, etc.
Still, it's pretty fun to see real landmarks in a game.
While it would be cool to drive by my place, it's much extra work for the develoe