Maybe, a local monopoly could be a cause for concern. But is the barrier to start a veterinary practice so high as to grant them a permanent monopoly?

In this specific case being discussed, it does seem the vet business was acquired by a PE company a few years ago:

https://www.mainebiz.biz/article/portland-animal-health-star...

Note that the vet doctors themselves may have ownership in the company itself.

The other option is that advanced surgeries for animals are so costly, and the proportion of people who have the ability to pay so low, that there does not exist sufficient demand for a new vet to want to open a new hospital. In short, pet owners simply cannot afford all the pet healthcare they want.