Because it is.
There's no 'ancient history' to Vegas, because it goes like this: House prepares for the Great War. Installs laser cannons in his tower. Great War happens, his laser cannons save vegas save for maybe a nuke blast at Black Mountain (air burst?) and the Devil's whatever out East and some other bombs far off. Everyone in Vegas celebrates for a hour but then die to rad storms (sic?) coming in. Vegas and the Mojave is a empty wasteland thereof, save for some tribals that'll become the three families.
Because the three families are utterly indoctrinated by House, we know nothing of their history other than that the Tops guys used to be nomadic (the boot riders), the White Gloves had a cannibal cave, and the Omertas were also nomadic, acting like the Indian Thuggees. The Jackals, Scorpions, Vipers seem to creep in between F1 and F2; the Khans creep in after F2.
The NCR follows them due to a mix of scouts reporting the dam and lake mead are fine, and to fight the raiders in the region, including the Khans. The followers come in at this time. BOS fled a bit before this, but not too much. House wakes up around this time and approaches the NCR at the dam after recruiting the three families, who seemed not to be directly targeted by the NCR.
The boomers left Vault 34 like 50? years before the game, Goodsprings is resettled by the NCR, Vault 3 opened up and was massacred, the Kings are less than 15 years old as well or so.
Jacobstown also seems to be relatively new, with Marcus setting it up due to the close but not immediate NCR presence. Everything else is boom-and-bust due to the NCR and the strip opening up again. Westside and so might have a bit of a older history but are really undercooked. The Highway man is here, implying the Chosen One came out this way, and the Vault Dweller technically? can comb the NV ruins in 1 or so but it's just urban terrain and urban random events.
It would be neat if we found the White glove's old cannibal cave, I guess, and a few more NCR bases along the main routes or that like, maybe ranger station charlie was a repurposed old base and stuff like that.