Because it isn't quite that simple. Overprescription of antibiotics isn't usually done only for the placebo effects, but other social reasons as well. A significant subset of patients react poorly to "it's probably viral and so we probably can't do anything". As bad as antibiotic overuse is, lying to patients and fraudulently prescribing a treatment is still much worse than giving someone antibiotics in the 5% chance that something is bacterial.

I don't think that lying to patients to falsely reinforce their untrue beliefs is a solution to the problem -- it'll probably create an additional problem by reinforcing beliefs of the legitimacy of pseudoscience.