Comment by jandrewrogers

The practical problem is that filesystems and databases are highly optimized toward different objectives at an architectural level. You can treat one like the other but the performance will be comparatively terrible.

This is made even more difficult by the practical necessity of backward compatibility with standards like POSIX, SQL, et al in the same implementation that were never intended to be interchangeable at the implementation level.

In principle you could design a database to be used as an effective filesystem. In practice, the implementation wouldn't be very compatible with any other software you need to work with and the ability to integrate matters.