Uber won't last long enough to acquire a traditional automaker before technology creates a death spiral for those automakers. I'm wrong if they can find additional investors willing to gamble on the expected duration of time till that happens (~5 years at least).

Tesla has a product people will buy today (with ~25% margins!), as well as a team to transition to ridesharing in the future. They're collecting millions of miles per day of autopilot data and that'll only increase with the model 3 rollout.