Because, short of hospitalization for every infection, there’s no way to do so, even if you were to assign an antibiotic police. So, the next best thing, after weighing the risks, is to try to shorten the regimen so it’s easy and convenient for the patient, and get a proper prescription from the doctor (since there’s all sorts of antibiotics) as to greatly mitigate overuse and to educate the patient per infection about the consequences of stopping half way through: e.g. “You’ll be increasing the risk of developing a strain of bacteria that we won’t be able to fight with antibotics, from this very infection.”
See that? I employed common sense.