Its not unusual to be 20 years old and not have a girlfriend (or boyfriend).

Family relationships change when people get older.

Also, it sounds like people are just contacting you when they need something. That's no good. Stop being available for helping them so much. If they don't want to hang out just for the sake of friendship and being together, then they aren't real friends. It is better to be alone than be with people who are just using you for help. I have had to be alone with no social life, because I had to cut out people from my life. It was hard, but that's what I had to do. When people know you aren't afraid of being alone, that gives you more power with them.