>but guns were expensive and most armies wouldnt have been equipped with them

Actually, firearm use in China was pretty prolific by the time of the Ming Dynasty. Your argument about higher cost can be applied to crossbows as well, but obviously they were still issued and deployed on a large scale despite this.

Armour (even fabric or leather based armour) is expensive as well, so it was mostly worn by elite or noble troops who could afford it. Also keep in mind that the Chinese and Koreans made heavier use of peasant levies/conscripts in their militaries relative to their European/Japanese counterparts (who mostly depended on smaller armies of professional/semi-professional soldiers), so obviously had a harder time outfitting their entire forces with armour.

>Seems more like the institutions in both China and Korea were so eroded by corruption and incompetence that soldiers were regarded as expendable and thus werent provided with armor.

I don't know about Korea, but in the case of Qing China that is sort of true. In the late-1700s, the Qing emperor enacted a freeze on troop salaries, so soldier pay didn't keep up with inflation. The Banner armies in particular suffered from lack of armed conflict in the relative peaceful period between the Qing invasion of Vietnam (1789) and the 1st Opium War (1839).

The lack of fighting led to idleness and neglect in maintaining combat effectiveness (instead of training, Banner soldiers spent their time drinking and gambling). Also with the way the Qing military worked, Banner troops got paid much more when on campaign. Since they were also prohibited from doing other jobs or running businesses, in peacetime they had no means of income other than the small stipend (2-4 taels of silver per month) provided by the imperial court that was more for maintaining equipment and horses (Banner troops had to buy their own). With the pay freeze and inflation, lack of combat and the usual loot gained from pillaging the enemy, most Banner troops found themselves in an economic tough spot. Despite their reputation as the Qing's elite troops many were impoverished by standards of the time, and their situation was passed on to their children as their position/duties in the Banner forces were hereditary. I won't be surprised if these Bannermen were regularly pawning off their father or grandfather's armour and weapons for booze money as a result of their poverty.

The Green Standard forces weren't any better, facing serious issues in desertion, corruption (officers frequently stole the pay of their soldiers, or straight made up the number of recruits they had to embezzle money from the imperial court), and lack of funding.