Since cloud storage solutions come preinstalled with the operating system. E.g. Finder + iCloud, Windows file manager + OneDrive.

That is because the OS developer wants to push their product, not because it is something users want.

A third party cannot decide to provide something users do not want. Maybe they can get it preinstalled on Windows PCs or ANdroid phones. I cannot see MS (Or Google) being very happy with that.

Those come with the operating systems. They are not tied to the default file managers.

Since always. Technically, any file explorer is accessing distributed storage. First the multiple storage-devices in your local system, later the different storage in your local network, which then grew to support remote servers like FTP, Webdav, etc. And modern file managers have for some years now also support for cloud-storage like OneDrive or Dropbox. Nothing new here, except being an app build from the ground up for supporting this specific targets.

> First the multiple storage-devices in your local system, later the different storage in your local network, which then grew to support remote servers like FTP, Webdav, etc.

These are not distributed file systems.

> And modern file managers have for some years now also support for cloud-storage like OneDrive or Dropbox.

OneDrive and Dropbox are not tied to any file manager.