you know, i'm not really sure why ammo doesn't go bad. plenty of proof that it can last a long time, but it seems like there's also some indication that it can go bad.
when i asked vhitavouri why they only sold their double-base 'high energy' powders in 1lb cans, they said because it goes bad faster than regular powders and they figured reloaders might not use 5 or 8 lbs before it would go bad. also lots of threads in reloading about how to tell when powder has gone bad.
so presumably it's heat and oxygen that make it go bad, and there's potentially some of both when it's a loaded cartridge. showing i'm not a physicist here, but it's not clear to me why the military would need sealants around the bullet and primer to keep out water, but not to keep out air.
also, some countries seem to think it goes bad faster than others (e.g. UK vs US disposal rates)