My understanding is you have to become a qualified vendor with PSPC. My understanding is it's a pretty arduous task to try to do that, and a lot of people don't want to do it. Whether you're an independent or whether you're a firm, a lot of people are okay with having a middleman to navigate those potential hurdles for them.
It's typically by choice. The ones that want to do so become qualified vendors and have the opportunity to go directly to the government.