The US spends much of its defense budget on building expensive high-tech toys and maintaining 11 carrier strike groups, because it's military priorities are, in decreasing priority:

* Making sure everyone loses a MAD nuclear war

* Maintaining undisputed naval dominance in five oceans.

* Bombing people on its imperial adventures all around the world.

* Offering security and protection in exchange for military and economic and political obeisance from its vassals and client states. [1]

North Korea spends much of theirs on artillery shells, because it's military priorities are, in decreasing priority:

* Make themselves unattackable due to its small nuclear arsenal.

* Make themselves unprofitable to attack, due to holding a conventional-artillery Sword of Damocles over South Korea's cities.

* Being able to resist a ground invasion along a clearly-defined border.

It doesn't maintain more than a mothball air force, and a rag-tag brown-water navy, because both will be blown out of the sky, or the water within days of a shooting war breaking out.

It turns out that air forces and navies are very expensive to operate. Artillery, not so much, any asshole with a basic understanding of a lathe and undergrad chemistry knowledge could conceivably run a munitions plant.

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[1] The promise of security and protection turns out to have been written on tissue paper, because it can't even defend its own assets in a shooting war with a bankrupt regional power.