Yeah, it's not like Fallout has ever really qualified as "hard" science fiction. There's some trappings of some hard science, but it's only ever been for atmospheric purposes (reading the descriptions of the FEV experiments in the old Fallouts, for example - just a bunch of scientific terms to give the semblance of actual science without actually doing so.)
The entire premise of the universe, from the state of the Wasteland, to the retro stylings of Pre-War civilization, is firmly grounded in the principles of "soft" science fiction (like Star Trek, Star Wars, etc.) Where you take the end result and atmosphere you're looking for, and then work backwards from there to come up with supporting details. (ie, you need spaceships that can break the speed of light and travel great distances for the purposes of the story - so you say it's all done with "Warp Drives" and the like and leave it at that.)
"Hard" science fiction generally works forward from a scientific principle and works forward from there to determine the realistic results of said science, and the stories that could unfold from that concept. Like Arthur C. Clarke envisioning what spaceship sailing on the solar winds would be like, and then coming up with a story about that. The differences get a bit more complex than that, but as a gross oversimplification...
"Soft" science fiction breaks down when you try to apply the standards of "hard" science fiction to it. If Luke Skywalker actually travelled faster than the speed of light on his way to meet Yoda at Degobah, he would have arrived decades after the Empire had totally destroyed the Rebellion, for example. And then we wouldn't have even had a story to tell.
And anyway, none of that really matters anyway in regards to this subject. The only firm details we know of Pre-War Fallout (besides what timeline events have been previously laid out and can be perused in the Wiki) is that by 2077 the world looked remarkably like an idealized version of a 1950's vision of an Atomic Age Utopia. There's never been any details that pointed at society remaining stagnant and watching Leave it to Beaver for 120 years and not advancing in any way or experiencing typical cyclical changes. So to say that there's a problem with a detail that isn't even delineated within the Fallout universe in any way is sort of irrelevant, isn't it?
If you think it's unrealistic that society didn't change at all in 120 years, then you don't really have a problem, right? Because that's not what happened, it's never been said in any of the games that's how things worked out. All we know is the end result, not the specific road the world took to get there.