Hello! Don't know why I can't turn on saying "Siri" instead of "Hey Siri" on my 6th generation iPad? At WWDC they didn't say anything about device incompatibility.
The fine print on the iPadOS 17 product page says this:
Available in English (Australia, Canada, United Kingdom, U.S.) on iPad and on AirPods Pro (2nd generation).
Does this match your setup?