It takes quite a bit of work to maintain nuclear warheads. All active US weapons contain plutonium 239, which has a half life of 24,100 years. It's radioactive by alpha decay, which leads to changes in the material properties due to energetic collisions and the buildup of microscopic helium bubbles (alpha particles are merely ionized helium nuclei, so stopped alpha particles become helium). Since the US stopped testing actual nuclear warheads in the early 1990s, it takes a great deal of indirect theoretical and experimental evidence to make sure that nuclear warheads are reliable without live fire tests. That's part of "stockpile stewardship." [1] If the plutonium has deviated too far from its original mechanical behavior, it would need to be removed from warheads, purified, and remanufactured into replacements that match the original specs. And again, the rebuilt components need to be reliable but they can't actually be tested via explosion.

US weapons also rely on tritium gas "boosting" to operate reliably and efficiently [2], and tritium decays with only a 12.3 year half life. The gas reservoirs of weapons need their tritium replaced at significantly shorter intervals. Even manufacturing enough tritium to maintain the stockpile has become a challenge because the US has retired its Cold War era weapons-material reactors that used to operate at Hanford and Savannah River. Currently the US uses a power reactor owned by the Tennessee Valley Authority to make tritium for weapons [3].

It's possible to make nuclear weapons (even thermonuclear weapons) with only uranium 235 for fissile material and no stored tritium. Such weapons could last a much longer time without active maintenance, since U-235 decays thousands of times slower than Pu-239. However, they would be larger and heavier for the same explosive yield, which complicates delivery. They would also lose certain safety features. Finally, without being able to perform full scale tests, it is doubtful that the US would have the confidence to replace its current high-maintenance weapons stockpile with a new generation of low-maintenance weapons.

[1] https://en.wikipedia.org/wiki/Stockpile_stewardship

[2] https://nuclearweaponarchive.org/Nwfaq/Nfaq4-3.html#Nfaq4.3....

[3] https://www.wvlt.tv/2022/05/24/watts-bar-lone-source-nuclear... "Watts Bar lone source of a nuclear weapon material; TVA increasing production"