The only difference I can see is multiuser support of which Siri works just fine for me controlling the matter devices in the house that are on my partners Apple account. I can't justify buying a product just for multi user support and have another bulky device sitting in the house.
Apple would've been better off putting a FaceID camera in a Homepod mini or Homepod to get the same functionality.
The Apple TV and Homepod mini already support Matter.
An old iPad on a stand on the table in the breakfast nook is conceptually fine as a Matter controller until someone picks it up and leaves with it, or unplugs it and lets it run out of battery.
Apple forbids this kind of use because it defiles the majesty of their garden and its walls (or something), but that's just Apple doing Apple things.
Expectation of usage. An iPad designer assumes it will be taken all around, leave the house, put in a book bag, etc.
While an old iPad being used as an entertainment management surface won’t, that’s not the prime use case intended for iPad.
Apple decided to focus on products whose primary purpose is a stationary device, like a speaker or a TV-attached device. Sure, you can move them place to place, but most won’t.
And the usefulness (or lack thereof) of the functionality can never be tested by real people, because at Apple: Design is everything.
hasn't apple been shipping thread radios in devices for a few years now, with no actual use for them? the only thing blocking iPads from being a home hub for managing matter protocol devices is that apple hasn't turned that feature on.