The reason is because these people have little, if ANY, teaching experience. Let alone formal experience. Aside from interns and first-year residents, pretty much every clinician at a vet school has some degree of teaching experience or else they would not have been hired in the first place. Many have been mentoring everyone from green first-years in lecture halls to on-the-job rotations students for years and years. That is why they are called TEACHING hospitals. That is my main concern with picking Joe Schmo, DVM down the street, even if he is a nice person and has maybe mentored a handful of new grads. Teaching students correctly so they maintain as much knowledge as possible is one of the hardest things to do, and not something most people can do right off the bat. It also takes a LOT of patience and time - something that a teaching hospital can afford, but a private business that needs to keep itself as efficient as possible likely cannot.
In addition to the above, what about all the rotations outside of basic "community practice/internal medicine" such as radiology, dermatology, neurology, cardiology, anesthesia, pathology, ortho vs soft tissue surg, etc? Are we expecting these general practitioner vets to be as knowledgeable about all these subjects in practice as a board-certified specialist at a vet school who runs the individual rotation? Sure, they are probably ok to capable of most - being a GP is being a jack of all trades after all. But there are going to be significant gaps because of this both in terms of knowledge and practice.