Because the 'United Systems Military' effectively covers the same stuff. There would be no United States of America, if you've got unified world governments (backed up by a certain cahracter's comments of what Earth, collectively,ewas meant to be liked). For the same reasons, Earth is spoken of as virtually unpopulated or with very few people of any worth still on. Weyland-Yutani is invalidated by a character similarly saying they no longer exist. What did that story do?