Because there is no reason for the manufacturer to sell to Uber and Uber has no value proposition if driverless cars become real. If you are the builder of driverless cars why would you want Uber to dominate the market and be in a position to eventually dictate terms to you when you can instead sell at a premium to Uber's desperate competitors or just create your own car agency and laugh all the way to the bank as Uber becomes the next WebVan...