I mean it could be part of the OS. It would be kind of cool if your OS could have an API to encrypt private user data with some key that only the other persons OS can decode.

Chat apps could then delegate security to the OS, rather than having to trust that the app developer has a security team that knows what they're doing.

You could even take it one step further and have the plaintext of the users messages secret from the app - the OS would draw them direct to the screen. A bit like opaque cross-origin data in webpages is secret from JavaScript on the page.