A few imo-points.
1. computer games are abstractions
2. unless a game claims to be faithful simulation, only internal consistency really matters
In Fallout a lot of things can be easily explained away with "science!" tag since the apocalypse occurred in 2077, not yesterday.
Bottle cap as a main currency are perfectly plausible within the settings, they´re backed up by fresh water reserves in the Hub.
So is the gang of Elvis impersonators. "Conversion" to tribalism/gangism is one of the natural ways how to deal with the state Fallout world´s in, sharing the same visual identity strengthens sense of unity, resources are scarce, so if someone bumps on rock´n´roll school full of leather jackets and hair gel…
Etc.
What abstractions find their way into the game is influenced by design (genre, settings), artistic license, economical and technical reasons. The game´s foci are exploration (lots of locations, open ended gameplay), quests (multiple solutions, interconnectedness, consequences), faction interplay, character defining/building
and survival.
Details are thus distributed between more aspects of the game and detailing one aspect significantly more than the others would shift the game´s intended overall balance.
That said, people´s need for art wasn´t omitted entirely. There are radios, posters, a guy who´s running out of inspiration and player has to make photos for him and in one quest player has to recruit performers for casino theater, I´m sure there are other examples. Given the scope and focus of the game, that isn´t so bad.
The relative lack of colours is to be subscribed more to an artistic license than to failed realism check, and goes well with the game´s intended mood and morality grayness (Legion is dangerously close to cartoonish evil, but that´s besides the point here). For now, for those who wish more colours in their F:NV, there´s a choice between Fellout NV, Nevada Skies and URWL for New Vegas - Realistic Lighting mods (personally I don´t like either).
For a visually more vibrant Fallout title, I recommend Wasteland
.