Why do women ask "Why are you still single?" To me?

Simply put. I have been approached by a lot of girls across the scale of attractiveness and a lot ask me after talking with me for a while "Why are you still single?" Or "Do you have a girlfriend?" And when I tell them I don't they seem shocked or in disbelief, like they don't believe me.

I feel like something is seriously wrong with me if I got women asking me that, but can't seem to get far in terms of dating to the relationship status. It's pretty discouraging.
Why do women ask "Why are you still single?" To me?
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