Comment by inigyou

We do have a single database, it's the filesystem. But quite importantly it's a very composable kind of database, because you can "section off" a part of the database, calling it a folder, and the folder looks like a whole filesystem of its own. Working in the folder is like working in a separate database that's visible from the outside but the outside isn't visible from the inside unless you deliberately choose to go out through the escape hatch. SQL isn't like that - it's a flat namespace and that is like the original filesystems that didn't have folders. Those were usable when a computer was only big enough to hold a few activities at a time and when you had a stack of disks next to your computer, each with a different program's filesystem.

A flat namespace might actually work with an app-based OS model (like on android) where everything that happens is associated with an app ID and apps mostly can't talk to each other. But there's a reason nerds don't do nerd stuff on smartphones.