Apple's work on developing their own SOC (A13) have paid dividends for them. The same legendary chip designer (Jim Keller) designed the new Tesla chip after working for Apple.
For vertical integration to work you need to have funds to support it long term. Apple has it. Tesla - not that much (at least not in the current state)
Building one-off solution that will work good for your special use-case is hard, but in a doable territory. But cutting off fat that you don't need for your use-case is a one-off gain.
Building whole process that will keep on producing solutions that are better than what suppliers can offer, especially in highly competitive area, is way way harder. That's where the real money are spent.
From the video presentation, it did not seem to be a "one-off" effort. They mentioned that V2 was already 50% complete and will continue to be developed over time to avoid complacency and stay competitive while also keeping the cost at a manageable level.
I get what you're saying though. But if they manage to pull off the Robotaxi fleet, then $$$ won't be much of an issue.
Musk's ego plays a lot into this for better or worse.