Welcome to Vault 101!
The Fallout world is an anachronistic setting historically divergent from our own and fundamentally different from our universe in how the laws of science work. The base concept for the setting is a 1950s World of Tomorrow, a future as envisioned through the lens of the Atomic and jet Ages. The Fallout world is more or less what Americans of the 1950s thought things would be like in a future decimated by nuclear war.
The Fallout world is home to hovering housecleaning robots, and the use of laser guns is the norm. Automobiles look like Motorama concept vehicles from the 1950s: massive tail-finned and chromed behemoths, yet powered by nuclear fusion engines. While there are many desktop computers similar to our own (albeit on a par with those which became available to us in the 1980s, with primitive graphics and apparently tiny memory capacities), the major computers are still giant banks of machines and use reel-to-reel tape storage. However, the pinnacle models somehow are more powerful than our universe’s supercomputers. Clothing styles, architecture, building interiors and furnishings remained heavily influenced by the culture of the American 1950s, and popular styles of this period such as art-deco and futurism remained prevailent. Posters and signage also largely hearken to this decade. Radio, rather than television, remains the most common mass media, and food products are based on those popularized in the TV-dinner era (boxed macaroni and cheese, canned meat, Salisbury Steak TV dinners, etc.).
(source: Fallout Wiki)