Oh but it is - Macs are often used for, say, video editing. SGI realized the importance of filesystem performance in this application space and developed XFS (which Apple could adopt easily) and GRIO.
It would be intensely difficult technically for Apple to port XFS (perhaps not as bad as the rewrite to make it actually use Linux's VFS instead of implementing its own). It's also GPL — never going to happen for that reason alone.
As a longtime XFS user, I can tell you that it has pathological problems when used anywhere near a user's desktop: unlinking is abysmally slow (and even worse under rm -R). It also has a habit of delaying flush for way too long, both using up piles of kernel memory and making itself extremely vulnerable to power loss.
SGI has the full copyright to the version they initially released, but modern XFS has spent 8 years getting integrated into Linux by many independent contributors.
