That's an interesting one. As a psychotherapist from Germany I can say we're definitely not low paid, but it is much less than other academic professions, and especially in relation to the time it takes to get qualified (roughly 10 years) and the cost of approbation itself (varies from 30k-160k, and that's in a country where education usually is free) it's really not a good fit for someone who is very financially motivated. (Ironically because of the high upfront cost the job tends to attract people from well endowed backgrounds though.)

I think like in many helping professions we have a majority of very idealistic people who don't negotiate very well. Employers get away with way too much because refusal at our side at first only ever hurts the patients, so we kinda keep up with it. Maybe something similar is happening in the professions that are in my mind actually the most underpaid for their time, and that's nursing and care work of all sorts.