NTFS, HFS, NFS, MSDOSFS, ... So many sources of file system drivers were release with the exception of APFS.
The released the code they wrote for compatibility with other people's file systems, and a filesystem Apple developed in the 80s. They didn't release the code for their own, proprietary (and non-ancient) filesystems. It would be nice if they did, but you can see why they didn't.