The same reason flights are always overbooked. They rather oversell and hope someone cancels than undersell.

The flipside of this is that they have no downtime between patients because demand is through the roof and there aren't enough doctors. Predicting how long each visit will take is fraught, plus emergent stuff may come up that throws off the already tight schedule. Better to have a long time in the waiting room than wait weeks/months longer to get an appointment at all