speaking as a north American gwailo (canada but essentially the same as the US culturally), they are "hot" because:
1 they were growing up always in a a room full of gwaipos being the prettiest girl and got most of the attention from the boys, they quickly learned how to use this to their advantage. It has been proven by research that Young girls (ages 3-5) who are pretty somehow know they are pretty from the non verbal signals they pick up from adults, men and women around them. As they get older they learn how to milk this.
2 Asians growing up in North America, both men and women, carry themselves differently, their demeanor is more confident, their posture is better, they speak with more confidence and have more finesse.
Around central I can look at an Asian man or woman, usually <30 years, and can tell by how they walk, dress, speak whether they are local or ABC.
Just my 2 miao.