In the US the veterinary field is closely tied to the practice of caring for livestock animals, increasing profitability, fattening them for slaughter, etc. In fact, when studying to be a vet, being vegan is highly discouraged and can remove work/educational opportunities if you're found out to be an "animal rights activist." Also the slaughter lobbying agencies are often propped up by vet agencies, who get to dictate what is or isn't a cruel method of slaughter.

Hopefully someone can remind me the details, but in CA there is a controversial method of slaughter involving, I think, massively raising the temperature until the animals are killed, and while the government recognizes it as controversial, it points to what the head vet association defines as humane, saying that this group of vets closely connected to the pork industry gets to decide how the government gets to regulate the slaughter/treatment of animals.

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Overall, yes the ethics of being a vet definitely align with veganism for some. But a lot of vets are more rural and used to the ag industry, while choosing to work on animals in association with the cruelest industries that use animals.