Autonomic neurologists are the relevant specialists. I invite you to look for one in your local area and see how rare they are. The last I checked, the Seattle metro area (and in fact the whole state of Washington) had precisely one board-certified autonomic neurologist. Diagnostic delay is, unsurprisingly, around six years. Combine that with the fact that all the equipment you need to do a basic POTS test is a pulse oximeter and a blood pressure cuff, and yeah, you're going to have people self-diagnosing, for good reason.