Holly is hotter and/or makes much more effort to look hot. So even if we assume normal Holly is as ugly as Stan (she isn't, she would look like the landwhale), she goes to the gym, changes her eating habits, changes her hair style, her clothes, even her glasses. Imagine Stan getting a proper hair cut, wearing something good and getting in shape (which he starts in chapter 13).
Holly is also better personality wise. She has issues, but still socializes and approaches people despite of them, while Stan doesn't even tries. He got dumped, so got Holly. She attends book fairs, talks with her work colleagues (basic stuff that Stan doesn't even manage) and makes a move on people.
There is certainly some doublestandard and unfair biological sexual norm, men are expected to succeed and women have a bit more leisure and can get away with being meek. Though even they have to pay for that and can easily end up lonely, so I wouldn't go to far in the rabbit hole to complain about how women having it better.