And there's that issue of what "replayability" means to different people. I've only played FO:NV like 1.5 times, because there's so little to explore and so much of it is linear. It just doesn't compel me to play again. FO3, on the other hand, I've made at least half a dozen characters in. Most of them never bothered with the main quest except where they happened to run across parts of it while exploring. NV, there's very little to do except the MQ and the sidequests it leads you to. (My second, partial playthrough was after I got the DLC. And I just went through it enough to get to the DLC. Still have the fifth one to do, actually.)
Of course, it's probably because my approach when playing these open-world/sandbox games is "pick a playstyle/skillset, and explore the wastes". Not "create an elaborate backstory and try to fit it into the world." They're just basic character outlines.... (like, in Skyrim, I've got "good, Imperial-side, stealth/archery/sword&board" and "neutral, Nord-side, heavy armor & two-hander" among my characters. Not "outcast farmer from Morrowind" or "wandering minstrel from Cyrodiil, fleeing an arranged marriage". )