Why isn't Fitness+ available on MacOS?

Apple Fitness+ - Apple lists four compatible devices: iPhones, iPads, Apple TVs, and Apple Watches.

It looks like using Fitness+ on an Apple TV requires connecting an iPhone or Apple Watch to it. In the same place where Apple says that, they say that "You can also stream Fitness+ workouts and meditations to AirPlay‑enabled TVs and devices."

In case you didn't know, your M4 MacBook Air supports a feature called AirPlay to Mac.

Continuity features and requirements for Apple devices - Apple Support

This means that you can set it up as an AirPlay Receiver, and use AirPlay to stream stuff from your iPhone to it. So, if you have an iPhone or Apple Watch, you could stream Fitness+ workarounds and meditations to your M4 MacBook Air. I don't know if that would give you everything that you get with the combination of an iPhone and an Apple TV set-top box, because I'm not an Apple Fitness+ subscriber.

This is a user forum. We can't answer any question as to "why" Apple made the decision they did.