i got another question
why are caps still a thing and the main currency
They shouldn't.
Fallout 1 in California caps were a currency that was backed with water.
Fallout 2 in California caps were phased out and NCR money became a thing and were backed by gold.
Fallout Tactics further east they used soda can tabs as currency because glass bottles with tin caps weren't as common.
Fallout 3 Washington D.C. uses caps because ????????
New Vegas uses a combination of NCR, Legion money and caps. NCR's gold supply was cut in half making NCR money less valuable so out of fear of NCR money being worthless people went back to caps after hearing stories how 150+ years ago people use to barter with caps. They base this on pure speculation and rumors.
Fallout 4 boston uses caps because ??????????????.
It should all be based on region. It would have literally made more sense for Fallout 4 to use subway tokens as currency instead of caps given how many tokens there are.
Fallout 3 could have used something other than caps. The brotherhood in that region no doubt could have implemented their own currency.