> managing blobs on a disk is precisely what filesystems are intended for.
A filesystem is doing much more, e.g. providing naming and management (directories, symlinks, access control, extended attributes, cache management, …) for files for manipulation by humans and applications, whereas RDBMs only need fixed-sized blocks of storage.
Some databases actually support using raw disks without a normal filesystem, which can have advantages by removing the extra layer of abstraction, e.g.: