Everything at Amazon is about creating scale.

Prime exists to get shipping volumes up far enough to pull things like trucking items daily from Vegas to Southern California, directly into the USPS system at the sorting facilities - and eventually direct deliveries.

The other thing - which lead to AWS, FBA, etc, was that each Amazon department had to make a globally usable API, even for (then!) inside facing stuff.

Combining those and you get a giant. Hopefully Bezos will stick around for a while yet - I can't see Amazon continuing to not maximize profits in exchange for volume very long after he's gone.