Uber don't need to invent self-driving themselves so long as whoever does invent self-driving cars sells them on the free market. Uber has a strong brand and loyal customers, and they could make out like bandits even if they have to buy the tech from someone else.
Their real problem is if some other company perfects self-driving cars and doesn't sell them. Perhaps Uber have decided that's unlikely, either because no one will or because a company that isn't looking at ride-sharing will.