My guess: because the OP forgot that to decode AV2/AV3 it matters not that it is in an AVIF container, it matters that the display system (browser, etc.) actually have the codec available to it that can decode the new format (AV2/AV3/etc.).
With no codec for a new compression algorithm, it does not matter if the browser can read the container, you still see the "broken image" icon on your display.
With no codec for a new compression algorithm, it does not matter if the browser can read the container, you still see the "broken image" icon on your display.