Because Bethesda didn’t care to know how nuclear bombs work.

In real life, 200 years after a nuclear bomb detonation, it would look more like this:

or

and less like

The thing about radiation is that the rates of it deplete very quickly. Even if we had huge amounts of nukes dropped everywhere, 90% of the emitted radiation would be depleted after 7 hours. This means that if the level of radiation is at 1000 rads/hour, after 7 hours it would be 100 rads/hour. After another 7 hours, 10 rads/hour.

If we keep doing this, then after roughly 100 years, 94 to be precise, the radiation is harmless. It would only be 40% higher than current US background radiation.

Nukes aren’t so much instant XK class end-of-the-world bombs like so many people believe, but as General MacArthur put it “really big grenades”.

However, Bethesda wants to create a post-apocalyptic world with the premise of a nuclear fallout, so they can choose to ignore physics and do what they like because it’s a game.

Thanks to someone in the comments for reminding me about nuclear winters. If the nukes were widespread and powerful, then a nuclear winter could occur.

A nuclear winter would be caused by carbon particulates, produced by widespread fires, gathering in the air and blocking sunlight.

This may be a vastly bigger issue than the radiation fallout and could lead to a near extinction event. Though, as someone once said, “Life will, UHH, find a way”.

However, nuclear winter is still a hypothesis and has never actually been proven. For all we know, the smoke may never reach high enough until it is returned down to the ground as rain.