They do. That's what a modern file system is. The on disk data structures and related code is usually less complex than the higher level software features. It's

They do. That's what a modern file system is. The on disk data structures and related code is usually less complex than the higher level software features. It's just that unfortunately these layers aren't often exposed for use (ZFS is the exception). If I were writing a file system from scratch, and I have thought about this, I would make a base layer that did nothing more than provide a hardware independent interface to block devices plus B-tree and PATRICIA index implementation that includes block-level features such as RAID, snapshots, log structured updates, etc. Then on top of that you can either write a fast DBMS for a BeOS like experience, or a POSIX compatibility layer that makes a more traditional hierarchical interface.