Citing two sources, the article Korea under Japanese rule gives the following figures showing that very few Koreans who volunteered for enlistment into the Japanese army 1938 – 1943 were actually accepted.
Although some would have failed the medical, the acceptance seems far too low for this to be the only reason. Another possible reason is that the Japanese might have been worried about loyalty of these recruits, though even before World War II there were examples of Koreans who had been promoted to officers in the Imperial Japanese Army: Hong Sa-ik - later executed by the Allies for War Crimes - is one example, while Yi Un reached the rank of Colonel in 1935 and Lieutenant General in 1940. Five others also reached the rank of General by the end of the war.
Further confusing the picture is the forum post Koreans in the Imperial Japanese service which (citing specific dates and sources) seems to contradict the information in the Wikipedia article on enlistment, stating that enlistment targets prior to 1944 were not even close to being met. Referring to 1938,
Initially, the Army Special Enlistment System targeted "those who are better-off than average and ideologically solid"...However, it turned out that these wealthier Koreans were for the most part unwilling to enlist.
The article also mentions
blackmailing new graduates, putting pressure on families by detaining the father, or tricking them into enlisting by falsely informing them that doing so was just a formality.
Further, the article quotes An Outline of the History of the Army System by Masao Yamazaki that some (It’s not clear who the ‘some’ are) also insisted:
It is not appropriate to carry out this war only at the expense of Yamato people (ethnic Japanese) because if the war kills only Yamato people yet leaves Koreans, they will, together with their formidable power to reproduce themselves, pose a serious threat in the future
Despite the apparent contradictions in these two articles, they do both show that the number of Koreans serving in the Japanese army was low, which brings us back to the main question: why was this the case? Were perhaps those Koreans applying mostly from poor families and thus not the ones the Japanese wanted?
Note: I say 'apparent contradictions' as I'm not sure if I've correctly understood some the points in the second article cited - I'm no expert in this field.