Fallout 3 and beyond are not really sequels, they’re a completely different series set in the same universe.
I would argue they’re not even the same universe. While F1 had its share of of people living in post-war rubble, by F2 the world was mostly newly-build cities or primitive societies but there was a sense of progress, like having actual money (and by Tactics paper money was in everyday use). Then F3 comes and everyone is living in a pile of rubbish, with unreadable burnt pre-war books on their shelves like they want to pretend the world is how it used to be, nevermind that generations have passed, and everyone is back to trading in caps.