Simple, I tell you that in my message, if it’s genuine, I’ll use the word “apple” somewhere.
You tell me that you’ll use the word “banana”.
Provided no-one except you knows that my secret word is “apple”, you know the message came from me.
But it’s perfectly possible for you to fake a secret message and sign it “Love d1sxeyes, P.S. apples”, and so our approach only works between the two of us. A third party probably can’t even confirm that “apples” was the correct keyword, and even if they could, that can only prove that one of us wrote it, they can’t definitively say which one of us.
Now extrapolate this to using some more sensible mechanism than dictionary words.
You tell me that you’ll use the word “banana”.
Provided no-one except you knows that my secret word is “apple”, you know the message came from me.
But it’s perfectly possible for you to fake a secret message and sign it “Love d1sxeyes, P.S. apples”, and so our approach only works between the two of us. A third party probably can’t even confirm that “apples” was the correct keyword, and even if they could, that can only prove that one of us wrote it, they can’t definitively say which one of us.
Now extrapolate this to using some more sensible mechanism than dictionary words.