When I checked out the curricula of the veterinary colleges in the US, I found that the average time nutrition courses take up is 2-3 credit hours, often only during one semester. That's not a whole lot of time.
What's even sadder is that these nutrition courses aren't even focused on specific species - they include dogs, cats, poultry, horses, cattle, swine, sheep and so on. It's more of a broad overview of nutrition in general rather than discussing specific needs. (I've completed such a course.)
Those vets who actually have a solid knowledge of nutrition (which are very few, the majority relies on information from companies like Hill's etc.), have done their own research and completed further studies on the subject.
I agree that feeding is, or should be a huge part in caring for animals, but most people are pretty clueless in that regard and just rely on whatever BS the pet food industry feeds them.