Because the danger posed by a fairly low energy pressure vessel is highly related to it's failure mode. That's why OSHA has rules about what compressed air pipes can be made of--it's not about the pressure resistance, it's about what will happen if one fails.
It's likewise why most military boom is mostly not actually boom. With artillery you obviously need a very tough case, but standard aircraft-dropped iron bombs are mostly that: iron. They don't need that kind of strength except specialized bunker-busters, they're built that way because for a given weight of bomb you'll do more damage by throwing bits of bomb casing from a smaller charge than from a bigger charge without the fragments.
It's likewise why most military boom is mostly not actually boom. With artillery you obviously need a very tough case, but standard aircraft-dropped iron bombs are mostly that: iron. They don't need that kind of strength except specialized bunker-busters, they're built that way because for a given weight of bomb you'll do more damage by throwing bits of bomb casing from a smaller charge than from a bigger charge without the fragments.