Because knowing something and doubting something are two different things.
Using your "logic" then: Would Martin want to win an argument by admitting that he gave his opinions on things he knows nothing about, I seriously doubt it.
That's indeed a question - a silly one but a question nevertheless - and, regardless if you doubt it or not, the answer would be: No, Martin does not want to win an argument, simply because no argument to be won exists outside your head.
How anybody can win an argument by admitting that he is talking about something he knows nothing about is actually beyond any sane person's comprehension.
Btw your mindset about "wanting to win an argument" is very telling.