I’m sure if they could pay Koreans to do the work they would. The issue is that they’re too expensive. Korea has a highly educated population with extremely fierce competition to get into the best universities (the infamous CSAT) and the best jobs after graduation. Koreans who do not make it tend to move overseas where their education gives them an advantage over other immigrants for college and job spots. This process leaves very few available workers for many different low-skilled jobs (not just child care).