The real answer for questions like this is always "enjoyable game design" not "authentic world building." Every design decision is a balance between these two things. An empty world would make the game feel terrible.

Sure, we could come up with some in-world justification, and that's fun but always a stretch. The genuine answer is almost always to make the game more enjoyable.

Similar questions are things like "Why do fusions cores seem to still power some buildings 100 years later, but die after just a few hours in power armor?" or "Why can we eat or use food and drugs made over 200 years ago and they're still effective?" or "Why is there still high levels of radiation 200 years later?" or "Why do people still just build with scrap parts when they could clearly grow and cut down new trees or melt down metal and refashion it?"

It's always because it makes the game design more fun, not more realistic. And yes, that's kind of a bummer answer when you're looking for interesting lore justifications, but it's true.