It's just because it's easier...
Ideally you'd want to get a sample and send it off to pathology in order to choose the most effective antibiotic to treat the type of infection to prolong this problem we have with resistance.
However the patient doesn't want to book 2 or 3 sessions to see the Doctor and spend more money and they want a quick fix.
Doctor gives in to keep their patients happy and as a result all these beautiful broad spectrum antibiotics that are effective against a wide variety of infections become less and less effective with time as bacteria evolve (which is super fucking quick, like jesus christ).
Bit of a sticky situation.
I guess developing a system that could give you quicker results for the type of infection present would help immensely, but I'd have no fucking clue where to begin with that and now I'm just babbling.