Ask anyone in the pet food business: treats are much more profitable than food. The food is just the draw. The hope is that the customer will buy treats and accessories while shopping.
I'm much more concerned that my Yorkies insist on picking up a bag of CET chews when we visit the vet. Our vet is a mercenary and puts those bags in plain sight at the checkout counter so that I can't leave without buying one.
Seriously, there is no incentive for food companies to give kickbacks to vets to carry their food. Sure the companies benefit having their brand seen at the vet's office, but they don't need to pay the vets extra to carry it.