And since a filesystem is an ad hoc, informally-specified (or unspecified, how long did it take before linux could read ntfs?), slow (for some things: no indexes), bug-ridden (FAT 8.3 name limit, limited metadata such as file types), implementation of a database...

But filesystems have been in use long enough that most bugs have been fixed or worked around in the tools. And of course the guys who invented the 'filesystem' didn't have any databases to use instead.