One way to look at is that the third party GPU packagers have a different set of expertise. They generally build motherboards, GPU holder boards, RAM, and often monitors and mice as well. All of these product PCBs are cheaply made and don't depend on the performance of the latest TSMC node the way the GPU chips do, more about ticking feature boxes at the lowest cost.

So nvidia wouldn't have the connections or skillset to do budget manufacturing of low-cost holder boards the way ASUS or EVGA does. Plus with so many competitors angling to use the same nvidia GPU chips, nvidia collects all the margin regardless.