Why would any of those patients make up being in pain, let alone... most of them?

So they can spend money on treatment and waste their time obviously. /s

Opioid addicts frequently make up being in pain in order to obtain prescriptions. That's far less common in physical therapy patients but it does occasionally happen.

https://my.clevelandclinic.org/health/diseases/9833-munchaus...

It's not negligible, BECAUSE you are seeing dozens of patients a day. When you're a doctor, factoring the odds into your diagnosis will screw up your diagnosis BECAUSE you are exposed to the whole probability spectrum, not a quanta of it.

Using "chance of occurrence" is a major logical fallacy for diagnosis identification and can ruin the whole practice.

Besides the other responses so far, consider this: the consequences of mistakes about this are asymmetrical. A doctor could be exactly correct about the probability of "making it up", but the cost to the patient of a missed diagnosis is not balanced by the benefit of correctly dismissing other cases. A conscientious doctor could try to consciously account for this, but there's no feedback making them get it right (and pressures against, in our current reality).

Also, when a patient succeeds in getting a correct diagnosis after trying multiple doctors, the previous doctors generally will not find out about their mistake, so there's a bias against them actually learning the right probabilities: you can expect them to be overconfident on this score.