this is due to a 'conformation bias', most people dont know how to recognise someone as being on the spectrum unless it is very obvious. the people that are that obvious also, usually, dont have a boyfriend/girlfriend making outsiders think along the lines of "you see, another unlovable autistic"
aside from this, at the times where your relational status matters most (high school), your success is defined simply by who can get the most girls in the least time (not everywhere, but most environments think that way), making autistics behind on the game for several reasons.
long story short; it is just another stereotype, nothing more, nothing less
