They dont have to. They could make these games take place anywhere in the world far away from the strip. But as another user replied to me, they want the money because an exec thinks Fallout show popular so we'll say its around that.

It's like getting upset over the free choice in Knights of The Old Republic. It's a story that technically by movie POV, should have already happened exactly as it did in the history books, yet in-game we can do whatever the hell we want, up to killing pretty much whoever we want and just running purely dark side. Technically, we shouldn't be able to make our own choices and it should be a very linear game; but that's not what the developers wanted to do.

Same thing with Fallout.