Can you blame them? They were bitten once by relying too much on proprietary tech that they had to license from Mobileye who eventually pulled the plug. I can't imagine they are in any rush to repeat that process.

Bringing as much of the software and architecture in-house as possible shields them a bit more from that happening again.

I have no idea if that is the ultimate reason, or if it was even a significant reason, but it sure can't hurt!