The receiver is first party, the rest are third-party. Intentions and roles are different. Anyway, your argument could be extended to any E2EE protocol... And

The receiver is first party, the rest are third-party. Intentions and roles are different. Anyway, your argument could be extended to any E2EE protocol... And why we still use encryption? Because we usually trust the receiver and also the receiver can suffer from the leak of the message.