And also because in the first films the visions of the future only needed to serve the function of nightmare vision. They were terrifying but there was no story there, just a sense that nobody could stay still for long enough to rest or sleep because otherwise something would get them. It was an extrapolation of the themes of the first film, essentially, except with absolutely no escape, and all the imagery of skulls and endless flying machines and Terminators on the ground communicating that it would be impossible to live there. Eventually you would need to rest, and then, as with slowly stalking zombies and the creatures in Invasion of the Body Snatchers, they'll get you.
That's what I reckon anyway, and I also think it's inevitable that anyone trying to place a story within that futurescape is going to transform it into a more liveable place. That visceral terror of the future from the first two films works in glimpses, but not sustained over a two hour period unless your entire story is a computer game without any narrative beyond "run away from the scary shit".