Similarly on Android, WhatsApp suggests unencrypted backups to Google Drive by default.
Putting on my tinfoil hat, I am pretty sure that Google/Apple/Meta have some deal (successor to PRISM) where end-to-end encrypted messaging is tolerated as long as they have defaults that make it possible to access chats anyway. Apple not enabling ADP by default and WhatsApp doing Google Drive backups that are not end-to-end encrypted is the implementation. Since most people just use the defaults, it undermines security of people who care.
It's a 'win-win', the tech companies can wash their hands in innocence, the agencies get access to data, and phone users believe that they are chatting in a secure/private manner.