Again, without any real knowledge, this is all speculation on my part, but: I’m assuming multiple shipments for different customers are combined at different po

Again, without any real knowledge, this is all speculation on my part, but: I’m assuming multiple shipments for different customers are combined at different points en route, so Amazon is optimizing for cost by choosing the shipper that makes the most sense (for them) for all of those customers. Maybe it’s in fact trivial, but at Amazon’s scale I find it hard to imagine that any changes to their shipping algorithms are that easy, at least compared to the increased revenue that goes along with it, which in this case presumably is zero. If you’re willing to put up with all of Amazon’s other crap in exchange for convenience, you’re unlikely to drop them because they won’t allow you to pick the shipper, at least until someone else starts taking business away from them.