Modern US munitions have a mechanism that bricks the warhead after a certain amount of time from being fired (years). That way they retain use on the battlefield as a threat to enemy soldiers even if they don't properly detonate but will not pose a long term risk to civilians.
Maybe something useful happens to disable the fuse? The hundreds of pounds of high explosive are still hundreds of pounds of high explosive, in a rusty container, no?
While not universal, a lot of modern US munitions use polymer-bonded explosive (PBX) which is much less sensitive to accidental detonation from heat, shock, etc. so it is very unlikely to detonate from anything other than an intentional effort [1, 2]. They also use electro-mechanical fuzing systems, which are good but not great for safety, but are much better than mechanical fuzing [3]. All electric fuzing systems are the safest and are making their way into newer munitions [4].
The really useful explosives don't go off because of rough handling and don't become unstable over time - both very desirable properties to prevent handling accidents. Ideally, the fuse is the only thing capable of setting it off. So hopefully, the explosive will just sit there in it's shell and stay inert. Most WW II duds are dangerous because of their fuses, not the bulk explosives, as I understand it.
