Mad Max and Half-Life Similarities

This is the case with a lot of sequels when the original had some sort of world-ending premise. I think this is because post-apocalyptic scenarios and settings are more exciting than just rehashing the same story as the first movie/book/game, along with giving a sense of progression. And if you want to be cynical, it's because they had a setting/story lying around and just needed a popular character/franchise to leech it onto. But yes, generally, even if you save the world in the first movie/book/game, the second one tends to assume you didn't and that the world got severely boned in the meantime. Zombie movies have the progression from an isolated outbreak in the first (and possibly second) movie to a full blown zombie apocalypse in the sequels. Despite defeating Diablo in Diablo, in Diablo 2 the world is doomed. Speaking of doom: Doom 1 has an incident in an isolated base on Mars, Doom 2 is Hell on Earth. Literally.

So the transition from "small isolated skirmish in an otherwise normal world" into "post-apocalyptic wasteland ruled by monsters" is not uncommon. Resistance groups are par for the course, and cars made from scraps are just cool.