That's because they hope to date and interact with others of a similar level. Personally, most guys have male friends regardless of looks, but more so on similar personalities. Younger girls, some of them will pick out less attractive friends so they feel like they can be the hottest girl out there when they go out as a group. Not all girls do this, but more seem to do that than guys from what I've observed.
Probably to heighten their chances of encountering and interacting with even more attractive people.
Basically everyone wants to hang out with the most attractive people possible. However, usually only equally attractive people will have the means to be accepted into the group, whilst unattractive people won't fit in to the group of good looking people. So the outcome is your observation.