it is, but that's not the thing i'm taking issue with. the thing i take you with is that this question should be answered for you by playing any Fallout game! like, you'll have to dig through Fallout lore to find the in-universe, textual reason for the retrofuturism, but the games practically bang you over the head with a super sledge about the metatextual significance of the Atomic Era retrofuturism.

the games' setting is a clear satire of the nationism, imperialism, and hyper-capitalism of Atomic Era America (1945 to about 1963ish), working with a scenario that is emblematic of the era's nuclear warfare paranoia. the american culture that lived in the comfort and luxury of a booming post-war economy and was gripped by the paradoxical fear that atomic annihilation could strike at any moment, without warning. a culture informed by the lies their government were telling them about the survivability of nuclear war, leading them to believe that if they obeyed their government, prepared for the worst, and made sure to get their very own fallout shelter, they could survive an atomic attack unscathed. a culture that grafted together comfortable consumerism with a world on the brink of mutually assured destruction to create a happy, fear-based economy ready for the end of all things.

what Fallout points out is that if there was no political shift during the 1960s with civil rights and counterculture movements, and this regressive eschatological culture continued to persist through major technological developments, it's inevitable end point would be a world ending atomic war. like, that's one of the main points of the Fallout setting, and certainly one of the most defining things about this series. without that commentary being tied to the aesthetics of it, it would just be a bog standard post-apoc setting with a fun 1950s aesthetic (so basically Fallout 4)

if you're curious at all about the cultural impact atomic weapons and the Cold War had on atomic era america, i highly recommend the documentary The Atomic Café from 1982. the full film is available to watch on youtube for free:

watch it with an adblocker enabled so you don't get served an ad for geico ai generated perfume or whatever while learning about american war crimes