Well sure, but encrypted backups are still secure. What’s not secure (or private rather) is someone realizing they can’t have a backup of important chats and going back to Facebook Messenger.
Backups are a thing on Android, and they’re planned for iOS. It just hasn’t happened yet. People can choose what they want to backup and when they want disappearing messages turned on.
The process is a bit involved on mobile. Setting up a backup location, using a third party app to sync updates and deletions etc. It could be simplified by integrating with common cloud storage services (the encrypted file)
Also iOS doesn’t have backups at all last I checked. If you lose your phone the messages are toast