Why don't Vets allow payment plans?

I always wondered why don't vets allow you to pay back the bill in monthly installments. If you go to the doctor, specialist, or eye doctor, they will usually accept a payment plan. My dad has to have a shot in his eye that cost about 1,200 and his insurance will only cover $200. He needs this shot, otherwise he will go blind in one eye. He was actually going to stop treatment and choose to not see in one eye, but the Eye doctor specialist encouraged him not to and allow him to pay only $100 a month. He will be in hawk (debt) for the rest of his life, but at least they eye doctor was willing to work with him. Why don't vets do the same like doctors do with people.