Fallout nukes are portrayed as completely different from the real world ones and radiation in the fallout universe also works completely differently from the real world one.
If radiation would disperse as fast as the real world one then most puddles on the ground wouldn't be radioactive by 200 years, the white house crater wouldn't be so strongly radioactive that even in 200 years after the bombs the radiation kills humans in a few minutes, vault 87 only got hit by one nuke and humans die in a couple of seconds 200 years after the explosion, if we explode Megaton it delivers the same amount of radiation even if we wait for months/years and walk there again, the Glowing Sea kills unprotected humans in two minutes of exposure more than 200 years after the explosions, Long 15 and Dry Wells (totally deadly if the player tries to walk into the crater) will radiate the same no matter how long you wait, the Monongahela River and river bank are deadly from radiation too, (the river bank can give 300 rads/sec and the river itself can give from 600 to 2600 rads/sec) and Pittsburgh wasn't even hit by nukes, the radioactive fog and storms from Far Harbour happen since the bombs feel and still do more than 200 years later, you already mentioned the Glow, etc. if all these places that are not linked or have any waste sites or nuclear plant meltdowns near at all still have deadly radiation levels, then imagine when the bombs just fell.
Radiation in the Fallout universe definitely lasts way longer than in real world and doesn't follow the same rules. We can't just come from the real world and say that there wouldn't be any radiation there in a couple of weeks/years when there are still plenty of places that are deadly in the games centuries after it without any extraordinary reason for why they are still so radiated after so long.
And this is the very worst thing in Fallout: the SCIENCE! I think it's creepy that everything looks retrofuturistic, in a good way. But using B-movie science is an ode to ignorance. This should never be done.