Are we sure it can't? Because WhatsApp is closed-source, its GDrive backups are unencrypted and Facebook's whole profit model is based around snooping. Unless they make the app open-source, I'm not trusting them even with a grocery list. People act like E2E is the be-all and end-all but trusting an incredibly shady company on its word is not something I'm comfortable with.
GDrive backups are not readable by Facebook, they're readable by Google. End-to-end, if properly implemented is the be-all and end-all. Except for metadata, which is a problem, but a different one, and Facebook definitely abuses that. But they don't/can't read the contents of chat messages (for now).
It's not merely trusting that shady company, but also realizing that the news of FB not having E2E-encrypted messages would definitely make the news, you'd be aware of it.
Right.. consider what your adversary would be giving up by revealing such a secret, even if it was true. That alone provides a not-insubstantial amount of security.
> Are we sure it can't?
Google can remotely uninstall, and install a trojaned version of any app regardless of app signature on an official Android distribution.
No, there's a 1..2% chance of backdoor.
The real question is, why is Telegram more secure? There's a 100% chance it can read your group messages, because it says so on their documentation that describes the cloud encryption. There is no E2EE at all for groups. There is no E2EE at all for desktop. Together these mean E2EE are completely neutered and useless. I'm a privacy researcher and I don't use them at all. Why would an average joe?
Open source means anyone can audit and verify nothing was done after audit.
Moxie more or less audited WhatsApp's Signal protocol implementation, and people are right to be concerned about whether changes have been made since FB bought the app.