The issue becomes very difficult when you need to preserve message history reliably and allow account usage across devices. Because key management becomes tricky and clunky and there's no easy UX ways around it.

For ephemeral messaging like a SnapChat type use case, you can EASILY provide p2p encrypted. For services like Telegram and FacebookMessenger, their feature list requires cross device syncing, historical archiving, and those features don't play well with rotating keys, perfect forward secrecy, and other e2e encrypted messaging techniques.