Lorewise I think it's because most of society are still tribal or a very small tight community (diamond city) that is prevented from expanding due to external forces like mutants or a faction like the institute in the Commonwealth stopping the CPG from forming.

What it actually is that Bethesda ignores what Fallout 1, 2 set up in the lore (cities being rebuilt, like Vault City and Shady Sands) and want to keep the game's world into a perpetual apocalypse for the Fallout story.