Even before I had any idea what my romantic orientation was, I was a social outcast because I was "too much of a prude" and always got told that I needed to get laid. Only straight people who've never been around asexuals think we're anything like them. We get called out for being different. We're dehumanized for it, too. I certainly was. Never ending jokes about me being a 'robot girl' followed me for years. We aren't straight. We never have been. Why can't people just see that?
Real answer? Because accepting that we’re not straight would upset the model they’ve constructed about…the entire world, more or less, and force them to reevaluate and revise their worldview and the models through which they understand it, and that’s a scary prospect, so people who are unwilling to do it instead cling desperately to their old ways of thinking despite the overwhelming and mounting evidence that they’re wrong.