> Causality can't run from vet school being expensive to veterinary practices being profitable.

Yes, it can: expense of getting into the field limits competition.

Now, long-term steady-state eventually the market should adjust so that the people are willing to pay the up front cost because of the actual rewards, but real meaningful effects are often not long-run steady-state effects.

I hear a lot of failed vets become doctors.

Veterinary school is harder to get into and their is a lot of overlap in background materials. Med school has a lot of overlap so if somone desideds to swap carriers it’s an easy transition into a high paying job.

This may well be true. While there are hundreds of medical schools in the United States, there is only one vet school per state. And some, like Virginia and Maryland, combine - so there are in fact only about 30.