Frankly, the "need" for custom silicon seems self-imposed, with the idea of appearing to be a technology company first rather than a car / vehicle company.
Cruise automation was also for years working on custom silicon, I knew people there. Many people working on the custom devices didn't really believe in the mission statement either, but they were paid well, and got to do fun work, so they took the job.
What makes Rivian, or Tesla better at making the "normal" car pieces compared to Toyota, or Honda? The answer is they're really not better at those things and quite worse typically ; bad fit and finish, rattles, corroding suspension components, difficult to buy replacement parts, etc.
If these companies were truly about making electric cars available to all, a partnership with a car company that knows how to do the "regular car stuff" makes a LOT more sense.
Instead you have these companies that might be innovative in the drive train, electronics, batteries, and co-packaging who have to learn all the "hard" stuff normal car companies have been doing for a 100-years.
Now, instead of moving into a partnership with a regular car company, they're becoming hardware/software organizations making custom silicon with custom software.
Even doing custom silicon and the associated software takes YEARS of expertise to do it and not have a 1000 warts, not withtsanding going into a MOVING VEHICLE where the risks of making mistakes is life and limb.
So, my conclusion is this is more fancy smoke and mirrors to impress investors and the general public, but not in the best interest of end-users (people who buy vehicles to use as transportation).