"they've already been doing so nonstop for decades" - I would think it's not that complicated to make a nuclear bomb today, is it ? Technology has been there for almost 100 years already.
Your thinking would be wrong, then. Making nuclear weapons is ridiculously complicated, tedious, and requires access to loads of very specific technology.
I would actually love to read a bit about it. Like, let's say a reasonably sized developed country - say Australia for example, decides that making nukes it's a national priority. How long it would take them ?
