Because WhatsApp uses the Signal protocol design nobody ends up able to prove anything useful about the message history. To any outsider, including the WhatsApp server operators, the messages are opaque. To a participant, including you, the system delivers authenticated messages but deliberately doesn't sign them. So:
Alice knows she didn't send this message, so it's from Bob, but she can't prove to anyone else that she didn't just fake it.
Bob knows he sent the message but he can deny it and there's no proof.
Eve has no idea what the message was, even if Alice or Bob show her the real message she can't verify they aren't lying.
Alice knows she didn't send this message, so it's from Bob, but she can't prove to anyone else that she didn't just fake it.
Bob knows he sent the message but he can deny it and there's no proof.
Eve has no idea what the message was, even if Alice or Bob show her the real message she can't verify they aren't lying.