I believe this is partly caused by competition. As I'm sure you are aware, there are gazilion private clinics in one block competiting for business. You could be a sensible doctor and tell the patient with the common cold that it will get better naturally and send them home (or maybe prescribe them some painkillers). But giving them big piles of antibiotics makes the patients more content as they believe it is going to help. And content patients leads to better businesses.

Those small clinics live and die by local ajummas' word of mouth, so anything helps.

e.g.

"I went to Haeng-bok clinic and the doctor told me to just rest up but it just won't go away"

"Aigo, Unnie~ I went to Sa-rang clinic last time I caught a cold - you know the one at that intersection - and the doctor gave me all these pills. The cold was gone the next day!"