Because you can't hold a machine accountable, and if a pharmacist has to review the prescription anyway, the cost savings for just the dispensing part wouldn't be there considering the machines wouldn't be cheap.

I needed an emergency prescription the other night and arrived at the pharmacy three minutes before closing time. The pharmacist was able to verify my prescription, dispense the pills, check their work, and complete my transaction with a minute to spare. It would take a very long time to pay off a machine that handled just the dispensing part.