Google believes that its AI-driven chat app, Allo can do a better job, thereby leaving it up to users to delete their own messages.
And as you may already know, Google Assistant isn't quite a big believer in privacy, been that the app is tied to your Android phone, it request for the usually expected permissions: the ability to access your contacts, and more.
Which data will be kept by them indefinitely, or at least until you manually delete them, instead of keeping it on their servers for only a brief period of time.
The company initially considered keeping messages in a "transient" mode, but testing Allo revealed that the Smart Reply technology worked better if it had a history of user messages to learn from.
Albeit, Allo provides an Incognito Mode, which offers end-to-end encryption, meaning data remain scrambled and unreadable when they pass through the company servers, but it has to be enabled by a user.
Facebook owned WhatsApp, and Apple's iMessage, have taken a different approach, and doesn't leave it up to users to delete messages or choose to enable encryption, as messages are encrypted end-to-end by default.