Most people think that pretty girls can simply have things for the asking where others have to work for them or simply cannot obtain them.
When people go to the bars and they see men flocking to pay for their drinks, it's hard to not come to the conclusion that they live charmed lives.
And statistically, *THEY DO* - which is to say, they'll earn more money than their less attractive female peers, live longer (and healthier), attract partners etc. Jealousy begets bullying.
Of course beauty fades (and certainly women's beauty is generally considered connected to her youth), so some of this advantage goes away over time, but (for instance) if you get a high salary early on in life, you're more likely to have a higher salary in life later on life even if your beauty has faded.
And by the way, men can be particularly jealous of the power that female beauty brings (even when male beauty arguably brings more). After all, most men won't be able to "have" her, but most men will see what other men bring to her. How come she doesn't have to pay for her coffee, but she won't even let me pay for her coffee?
Jealousy begets bullying.