[1] Sometimes it's related to their store listing policies which are constantly changing, sometimes it's related to taxation in a specific country, sometimes it's related to laws in a specific country, sometimes it's actually related to software (on-device or web services!) they are forcing you to update/change etc. etc.
Apps can be local-first, reading and writing to files on the device. Then each user can choose their own syncing service like Syncthing or Resilio Sync, which runs in the background and automatically syncs those files to your other devices.
The restrictions imposed by the platform is why each app needs to have it's own half-implemented sync to it's own platform or some other cloud provider.
What it can’t do is grab your entire drive file list and contents the moment you sign in.