It works the same as industrial capacity for things like planes or artillery shells. Once you ramp up production you can produce lots very quickly, the US produced 70,000 between 1945 and 1990, averaging 30 per week with a peak rate around 54 per week; but building out the factories to ramp up takes a long time - for the first 10 years the US averaged 6 per week. Most of the US's nuclear production capacity was dismantled. With WW2 levels of hustle and disregard for safety, we could probably build new facilities in around a year or two. These efforts would be pulling resources from attempts to ramp up production of other wartime necessities. Also you don't just need to build the nukes, you also need to build adequate delivery systems, which are all advanced aerospace manufacturing.
If you eliminate your arsenal and then decide later you want it back, you're giving adversaries a lot of time to beat you to the punch, all the while advertising that you are pursuing nuclear as opposed to conventional weapons to fight your war.
If you eliminate your arsenal and then decide later you want it back, you're giving adversaries a lot of time to beat you to the punch, all the while advertising that you are pursuing nuclear as opposed to conventional weapons to fight your war.