The Short Version
Fitbit Air works with Pixel Watch only, leaving existing Fitbit device owners unable to use it without switching hardware.
Google built Fitbit Air to work exclusively with Pixel Watch, which means the millions of people already wearing a Fitbit device cannot use it without buying a completely different watch first. Google owns both brands, so this was a choice, not a technical limitation. Existing Fitbit users are the ones paying for that choice.