It is that pesky 99.9% effectiveness. That 0.1% that survived did so because they had some minor resistance. Rinse and repeat a few hundred thousand times and you have forced evolution. It doesn’t even take that long to happen in a population with the over-prescription rate we have had here. Something about the people in charge being undereducated religious ideologs who see expertise as a threat or fraud because experts make mistakes and learn from them.