You have to understand nuclear devices are very delicate, high maintenance items. You just cannot put them in a bunker underground and give them a look once in a while. They need to be monitored regularly (frequence varies according to the tech employed) by highly trained personnel using high tech equipment. A good chunk of the routine on an SSBN (nuclear armed submarine) revolves around making sure the nuclear weapons on board are working. If something’s wrong procedures have been devised to replace the whole missile at sea with the help of a submarine tender. SSBN’s are on 24/7 watch and cannot lose time re-entering harbor to do a missile swap.
Of course there’s literally a price to pay for this. Nuclear weapons are expensive to design, expensive to manufacture and expensive to maintain. After the fall of the Soviet Union Russia invested a disproportionate chunk of her budget to keep nuclear weapons working, at cost of leaving her conventional military to rot. Can a private group afford such costs? Perhaps, but I don’t think nobody ever did more than back of the envelope speculation.
Of course nuclear weapons have one huge bonus: they are the most powerful deterrent ever devised by man. Nikita Khrushchev understood this perfectly, so much he invested ungodly amounts of resources to close the nuclear gap with the US. US armed services were well aware around 1959 the US had a huge advantage over the USSR (which the Soviets desperately tried to close by fielding IRBM’s in Cuba, even at the cost of a serious political crisis) but played along the Soviets’ scheme. Mutually assured destruction (MAD) was seen as preferable to the temptation of launching a pre-emptive strike with unknown consequences. At the very worst nuclear weapons can be used as a Samson Option (Israel’s original nuclear doctrine): foreigners will think twice about invading your territory if you can take friends and foes together into nuclear oblivion.
One thing that should be considered is nuclear weapons can also be used as bargaining chips. Israel’s original Samson Option has evolved over time into a brutal yet effective diplomatic doctrine. Israeli leaders can force the US and Europe’s hand by threatening the use of nuclear weapons. Sure, they may be bluffing but do you really want to find out? Also Israel has been rumored to have offered a few Arab countries with good unofficial relationships (Kuwait, Qatar and Oman) a nuclear umbrella of protection in return for political benefits. Same thing about North Korea, which has been known to use her nuclear program as a bargaining tool to get fuel, food and money out of the US, Japan, ROK and even China.
So you see: a nuclear weapon is not just an instrument to destroy your enemies. It’s so mind-numbingly powerful it has other uses nobody could have envisioned in the late '40s.