Simple answer: People want an in-house built card that is a near silent blower style. (See Asus Turbo 970 reviews on newegg for the opposite).
Complicated answer: Nvidia coolers have always been desirable, so they can charge more for them. This also opens up lower pricing for competing vendors. Since Nvidia makes everything and doesn't pay itself a licensing fee for the tech, it could technically gouge out it's partners by lowering the price so low, that they can't compete unless they operate at a loss. So Nvidia keeps it's own parts high, and the partners still get to make profit. Now, that's not to say that they won't offer cards that are the same price as Nvidia's, they'll probably have more features and a higher clock, but you...