Financially, Amazon can currently benefit from Rivian selling vehicles at a loss. If Amazon purchases Rivian, Rivian's losses are now Amazon's. They would be out both the purchase price, and ongoing expenses.
If Rivian succeeds, they will be a good supplier. If Rivian will fail, Amazon doesnt want to be the one holding the bag.
Protecting the supply chain of parts for 10k vehicles is not worth much.