The radioactivity from nuclear fallout drops very rapidly — faster than most people realize. What this means is that if you are in an area where it is deadly-within-minutes or deadly-within-hours, within a week or two at most (and much less for many areas), that radioactivity will have dropped to "more than background but won't kill you immediately."

Which is to say: the fallout shelters were meant for short-term use. Two weeks for the worst-hit areas. A few days for many areas.

The goal of the shelter was not to live in it forever. It was to weather out the worst of the radioactivity, and then emerge to rebuild the nation. The goal of the Civil Defense programs was not to create a nation of isolated individuals living underground, but to preserve as many members of the national population as possible. It was also predicated on the idea that some kinds of national or local government would have survived as well and would be coordinate the rebuilding and relief effort.

Whether any of this is plausible depends on the scenarios you imagine, of course. But that was the goal of them.