Assuming that by attractive you mean pretty (dumb is not an attractive quality to me), in my opinion it is from the generalization that attractive women have always been able to rely on their physical attributes to have other people do things for them, and so they have never had to fight and struggle for whatever they need, be it a mate, job or any other kind of acceptance. If you’re plain, you have to develop smarts, personality or whatever else to level the playing field. That takes effort. And with that comes character. If a pretty girl took advantage of her beauty in high school and never had to put forth any effort (relying on a gaggle of boys to do her homework for her, do the majority of the science project, etc.) it would catch up with her later in life.
Having said that, good looks are hardly detrimental. I have never heard of someone saying that, all else being equal, he or she would never date/hire/befriend anyone because they were just too darned good looking.