I see CarPlay (and CarPlay Ultra) as being for auto makers who don't want to put in all the effort to design and drive a good proprietary UI (CarPlay is a godsend in cars with crappy UI, i.e. most of them).
Rivian is a luxury vehicle brand with a first-class UI/UX. I imagine going with their own first-class UI and CarPlay Ultra would be a mess; two separate interfaces for the same controls, but laid out differently. Makes a lot more sense they'd be working with Apple to integrate more Apple features into their own UI, rather than having to maintain two separate first-class UIs that are bound to have discrepancies.
And there's the more obvious answer that they want the entire driving experience to feel like a Rivian experience, given how important that's been for luxury EVs on the software side. Supporting a canned OS would make the vehicle "feel" the same as every other car that also supports it.
