Because our attempts at adapting to the demands placed on us by the broader NT culture re: socialization are often so bad. Case in point (from personal experience): the last girl I asked out on a date turned out to be taken. I did not know this because every time she found me sitting alone in the bar and invited me to hang out with the rest of the members of the class, we ended up comparing our experiences living in foreign countries or talking about law, things that are interesting, rather than talking about anything more "personal," which is not interesting. If I'd asked her point-blank whether she had a boyfriend (how I shudder to type that word), it would have interrupted the flow of the conversation which was inevitably comparing criminal juries in the Far East (me) to those in the Middle East (her).
I think the broader problem undergirding all of this is that we really don't respond to social cues. Or, rather, if we've learned how to respond to them at all, it's mostly book learning or observation that's devoid of any appreciation of the subtleties that NTs take for granted. A girl touches you...that means she's romantically interested, right? WRONG. And there's very little basis for me to know what she's thinking. It could be that she is, it could be that she wants to be friends, it could be that she thinks you're some sort of pitiful loner who needs to join the rest of the group, it could be that she wants to congratulate you for something (that happened to me...a girl who hadn't spoken to me for nearly three months and won't now grabbing my arm and dragging me away from everyone else because I'd just first-chaired a summary judgment hearing for the defense), it could be that she wants your help for some reason (also happened to me...my second chair holding hands with me because she wanted to get away from some creep who was hitting on her and no one else she knew was around), or it could be that she just wants you to get out of the way. You don't know, and there's no way for you to know, It's not as if anyone is carrying around a sign saying what she's really trying to do or would ever tell you outright.
"Flirting" is not an Aspie concept. And it's sad because so much of getting a serious date in this world depends on reading these subtleties. I've had, optimistically, four partners who were forthright enough to tell me exactly what they wanted. With everyone else, it was just guessing.