Why do you think people have to have boyfriend/ girlfriend relationships? Why can't it be we're fuck buddies or lovers or casual sex partners instead?

Many people don't- much of the world doesn't work in boyfriend/girlfriend terms, at least not the way I'm accustomed them being used.
Regardless, individuals work differently. I greatly enjoy both friendships with benefits and committed relationships.
Most importantly, many people simply don't find their needs met by casual encounters exclusively.
Additionally, there are myriad stigmas based on that sort of dynamic, some people just don't know there are other healthy options.

Step right up...