Writing a backup tool and supporting an OS properly is a lot more work than just writing portable software that can run on the portable standard APIs supported by an OS. People expect their backups to preserve file attributes, access control policies, etc.
Beyond traditional POSIX file attributes like ownership and access mode, there are POSIX ACLs. Linux also has extended attributes and SELinux policies. Windows has a different ACL model, and NFSv4 ACLs on Linux are more like those. A Mac has "resource forks" which have no real analogue in Linux or POSIX systems, and a backup/restore that ignores these would be very useless to most Mac users.
Beyond traditional POSIX file attributes like ownership and access mode, there are POSIX ACLs. Linux also has extended attributes and SELinux policies. Windows has a different ACL model, and NFSv4 ACLs on Linux are more like those. A Mac has "resource forks" which have no real analogue in Linux or POSIX systems, and a backup/restore that ignores these would be very useless to most Mac users.