Because people are cheap and stupid. Doing artificial gravity like the space station in 2001: A Space Odyssey requires building a really big space station (because of the large diameter needed to get 1g or close to it, and avoid problems with the force felt at your head being noticeably different than at your feet), which is expensive. It would cost tens of billions of dollars at least. We'd rather spend that money on building more nuclear weapons and on tax cuts for and giveaways to incumbent corporations.