Fallout isn't fantasy, it's science fiction.
Fantasy as a genre is all about the "fantastic". We take an otherwise mundane setting (even if it does have interesting geography or politics, that's not what I mean by mundane) and add a super-natural twist. Deities, magic, cults, rebellions are all part of the landscape. You also usually find common tropes such as evil overlords, swords and an adventure.
To enjoy a fantasy adventure there must therefore be suspended disbelief. The audience has to accept that magic and monsters exist. To say either is "realistic" therefore isn't really accurate. However to say one is "more realistic" than the other (ie. closer to reality) can be and in the case of the OP, I would agree, in general.
On the basis that if you removed the fantastic elements from the fantasy, leaving just the characters and the world they inhabit, it is most certainly a more realistic world that could exist (and in the case of medevil fantasy towns, actually did). I think part of the main difference is a cultural one. JRPGs tend to be how they are because of the japanese culture and mindset. Cuteness, children saving the world, swords the size of buildings, giant robots and so on. Western fantasy tends to involve chainmail, elves, dwarves, dungeons and dragons.