Most Helpful Girl
If she can't get who she wants and settles for the last resort.
Or if she overlooks his appearance and loves his personality more.
With the vast majority of our generation, its more likely to be the first one. Same with guys if they have no luck with getting women they desire. They settle for someone less attractive because of fear of being alone for the rest of their lives.2
Most Helpful Guy
Its because far less men are considered good looking. If you have 100 men and 100 women, say 20 men and 50 women are going to be considered at least somewhat attractive, the others are considered unattractive, if they all partner off obviously non-ugly women are going to be dating ugly men because more men are considered ugly.
Its also somewhat of a myth anyway. While obviously you do find some men dating much more attractive women, its not a very large proportion of them, the vast majority of people date someone around equally attractive as themselves, which is why ugly men find it very hard to date as they hugely outnumber ugly women.1