Most traditional OEMs in the automotive industry are integrators. They write a specification, and a Tier 1 supplier provides a black box matching that spec. (Tier 1 in turn provides a spec to its suppliers and integrates their parts)
This has several consequences
- Tier 1 suppliers are waiting on input/approval from OEM before proceeding with projects
- Tier 1 suppliers don't necessarily have the capital to do work "at risk", even if they could build the part without approval/specifications. (TBH - some do)
- Each layer of the supply chain lacks context on the whole project and product line and cannot achieve efficiencies outside of the scope of its contract.
These haven't really been a problem for mechanical parts and E/E parts that have well-defined functions and interfaces and have a lot of re-use from previous generations. It works really well with Kaizen (incremental innovation).
To outsource it, a traditional OEM would need to completely specify the behaviour of said self-driving system, baseline the specification, put out the requests for quotation, etc. Tier 1 then needs to analyse the spec, estimate it, break it down in to sub work packages, work with its suppliers, etc. From an optimisation point of view, this is really inefficient partitioning of the problem space. For greenfields development, an emergent specification via experimentation may be better - but that won't fit in traditional V-model sub-contract OEMs/Tier 1s use.
That flow doesn't need to be followed; the suppliers could raise/allocate capital and build the self-driving stack "at risk" - and this seems to be done (Tier IV, Waymo, etc). But as it's new technology, I assume Rivian think they can do better by themselves and can get the capital for the development as part of an integrated solution while they are smaller it might seem they should not waste limited capitial that way - but integration will save a lot of inefficiency in sharing specifications across boundaries, full system integration and deriving emergent specification via experimentation rather than some MBSE folly.