They wouldn't gain any strategic advantage over any of the competitors in their industry by owning a vehicle manufacturer.
The only value of EV delivery trucks for Amazon is helping their brand image and reducing their fuel and maintenance expenses. They can get that with any electric delivery truck manufacturer.
The only thing that owning Rivian would do is put pressure on their profits.
This is a somewhat funny thing to say about a company whose "business" has gradually come to include: B2C e-commerce, B2B e-commerce platforms, web hosting (AWS), brick and mortar grocery store chains (Whole Foods and Fresh/Go), pharmaceuticals, healthcare (One Medical), Twitch, streaming video games (Luna), film and television production (in-house and through its acquisition of MGM), consumer electronics (Kindle, Alexa, Fire), home security (Ring), robotics, and satellites (Kuiper).
How many of these were "related" to Amazon's business, which, as we all know, is selling books through the mail?
https://www.forbes.com/sites/moorinsights/2021/04/30/is-amaz...
Amazon sells very few things that cost as much as a car, and when they do, they don't manufacture them, because each transaction carries a lot of risk and overhead.
That's not to say they'd never acquire them, but the lack of business alignment makes it seem less likely.
Even UPS, who design their own delivery vehicles, contract out the manufacturing.