Blame the app developers, not google. They specifically added a backup/restore mode for device to device transfer, that bypasses backup blacklists[1]. However apps can still opt out by registering a backup agent, and returning no data.
[1] https://developer.android.com/identity/data/testingbackup
>Of course many app developers decided the Google cloud is too insecure, being not end-to-end encrypted
But so far as I can tell D2D transfers don't hit the cloud?
>For a D2D transfer, the Backup Manager Service queries your app for backup data and passes it directly to the Backup Manager Service on the new device, which loads it in to your app.
https://developer.android.com/identity/data/testingbackup
If your app is opting out of backup by implementing a custom backup agent that returns no data, it's pretty clear you're against user backups, period.
Google actively avoided providing a local, secure, and seamless backup or even an interface for 3rd party backup services to make users more dependent on Google cloud services. Of course many app developers decided the Google cloud is too insecure, being not end-to-end encrypted. And Google enables them by not giving the users ways to override those stupid decisions. This wouldn't have happened on PCs, where you can mostly just copy over the application's user directory.