Because like a PS4, macOS is considered a separate platform. (Even PS3 was, although those got free upgrades to PS4 when PS3 service was shut down). Platforms and subscription service are two separate things.

You buy the game (and latest expansion) once on each platform you play on. Then your service is the recurring fees. Some distributors give cross-platform access, but not all.

Better yet though, don't get the macOS copy of FFXIV. Quite frankly, it's garbage and even after they took it off the store and then added it back after "fixes" it's still garbage. You'll get the best results using BootCamp, and can just use the Windows version then. Otherwise, if you aren't afraid of developer tools and potentially having to use the Terminal, look into CrossOver or the project it's based on, Wine. (They do compile for OSX/macOS and the "Mac Port" is based on an archaic branch of wine that added DRM and not the good features.) You can then just play the Windows version in Wine, but you won't receive any official support for it should something go wrong or work unexpectedly.