say you want to store 10 million records. if you do this as a files in a say, one big directory (or even a sharded directory hiearchy), you have to update the parent directory (inode) when you create a new file (because the directory mtime needs to change) That's extra overhead vs what you can get with appending to a journaled database. POSIX encodes these specifications of what a file system is - so there isn't much ability to innovate here.

And yes - we work really hard to address the performance issues here, but there are constraints.