Why are girls always surprised that I’m single?

When girls I’m chatting to at parties or nightclubs or whatever ask me whether I’m single, they’re surprised when I say that I am. Why would that be surprising though? There are many possible reasons that a guy would be single, whether he’s single voluntarily or involuntarily. How could a girl be surprised that a guy is single, just from seeing him and talking to him for a few minutes? That’s surely not enough time to get to know someone and to form a judgment of them.
Why are girls always surprised that I’m single?
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