At least for OneDrive on Windows and iCloud on Mac, they're integrated with the OS and installed by default. There's no need for extra apps. It's not so good for Google Drive, Dropbox, Nextcloud, or other cloud storage services (or for that matter, OneDrive on Mac and iCloud on Windows) where you do need to grab and install an app.
I don't see "fragmentation" (as you put it) going away, because each service has its own way of authenticating users and accessing the services, and each OS has its own API for mounting or synchronizing files.
I don't see "fragmentation" (as you put it) going away, because each service has its own way of authenticating users and accessing the services, and each OS has its own API for mounting or synchronizing files.