Borderlands isn't post-apocalyptic. It's set on an inhospitable planet that the only reason people are even there is that treasure hunters and adventurers go to it following rumors of fame and fortune. I do love the idea of Borderlands, except, well, you said post-apocalyptic and that's not actually a bad description of what Borderlands feels like, and Borderlands totally has a bunch of Mad Max in its DNA. I think any movie of Borderlands would just end up being a Mad Max ripoff with alien monsters.