There's something pretty universal about making friends. Rarely, if ever will people come up to you if they don't know you and invite you out. I made a point of doing it when I saw someone sitting alone at the dining hall, but it's not something that happens often.

People you know don't invite you out because they don't know that you like them. Invite THEM to go catch a movie, or play pool, or grab food. Why on earth would someone want to hang out with you if they don't even know that you want to hang out with them?

Stop relying on other people to be friends with you, go out and be friends with other people. Be that guy.