It's not "Siri is smarter" but rather "Apple is working on minimizing cloud costs because it is entirely a cost for them."
Siri is scripted and limited to make it locally "smarter".
With Amazon and Alexa, everything goes to AWS because that's where the entirety of Alexa's processing happens. The hardware devices in the homes are "dumb" terminals for AWS with a voice interface. This allows Amazon to make use of AWS as much as it can and do something with "surplus" computing power that it has available on AWS.