I don't know how much it being set (in the show and the original games and new Vegas) in an arid dry location might impact your average building's longevity, and also at least in this case I just ultimately consider that the property is based on the setting for a D&D game iirc and in the end rule of cool wins out. Maybe unrealistic, or maybe the result of an alternate timeline that led to divergent construction standards from the constant threat of war that made all buildings be regulated to be capable of withstanding direct attack or something stupid that inadvertently had the effect of making newer structures last for hundreds of years longer. There's all kinds of hand-waving that could just easily be done by Bethesda at this point, they already introduced the brotherhood of steel in 76 much earlier than previously established in their own canon, might as well retcon basically anything.

Edit: New Vegas, for example, had a lore excuse and shows what I think is also reasonable decay under the circumstances. Earlier games were similarly more realistically destitute and broken down, except where it made sense. Bethesda likely just thought that running around a bunch of radioactive ash would be more boring.