my first response was 'but filesystems live on the disk' while databases are simply to set up in userspace. but then i realized that with FUSE and filesystem-image files and the right tooling the same flexibility can be achieved, and also, on the other side high performant databases want to live on the raw disk to be in full control of the write process to disk.

what remains is that the difference between a filesystem and a database is the API used to talk to it.