If they didn't want to be approached, they would find a way to leave the event or avoid it in the first place. A lot of times it is simple social anxiety. They are afraid that people will find out something about them; or rather, afraid that someone will confirm something they suspect about themselves (e.g., "I'm not interesting to other people.")

Those kinds of fears are generally unfounded, but it's difficult to prove to yourself that they're unfounded without actually interacting with people. But for those willing to interact with the wallflowers, they can be somewhat fearful of approaching someone with such a standoffish attitude, so it becomes a self-fulfilling feedback cycle.

Even just a little courage can go a long way. Walking across an open space can make you more approachable; for example, walking from somewhere on the edge of an activity like a dance to wherever beverages are being served can give others an opportunity to intercept you or approach you. It's not hard, it just takes some practice and the realization that the possibilities for negative consequences are much lower than the possibilities for positive outcomes.