Tim Bajarin, writing for Time:

Despite Amazon’s success, Apple has no apparent interest in copying the Echo. After talking with Apple executives, I’ve come away with the impression that they’re more interested in turning Siri into an omnipresent AI assistant across devices, rather than designing a single device specifically to serve as a Siri machine.

Interesting point. The Amazon Echo exists simply to listen for, and fulfill, Alexa requests. Every other Apple device serves many purposes and also serves the ecosystem. More bang for the buck.