The girl who is never approached

I feel so sad and forever alone because guys never approach me and I don't understand. I am 5'6 and weigh 125 lbs and I don't think I am ugly. I have a babyface and since I am thin, some people think I look younger than my age (I am 21 but some people think I look 14!). I even one time ended up talking to a guy and he assumed I had a boyfriend, when I said no, he was in shock. He said I am a very beautiful girl and that he sees ugly women with bfs all the time. Strangers (both men and women) tell me I am pretty/beautiful. I don't go to bars or clubs.

However, I am never the girl that is approached for dates or for my number in public anywhere! Unless men only approach ‘easy women’, and I guess I would not be classified as ‘easy.’ One time a guy cold approached and hit on a girl by asking for her number while I was standing right in front of her. This really hurt my esteem because I never get this type of attention. I keep doubting my attractiveness and that there is a possibility that i am not as attractive because no guys ever approach me for dates or to hit on me. They only say I am beautiful in the street and keep walking or when I happen to be in a conversation with a guy, he will tell me I am beautiful. Or, once in a blue moon, I see a guy stare at me openly and it looks like he is about to approach me but he never does it. I once caught a random guy staring at me in a train station and he looked away in embarrassment.

I don't understand why guys aren't interested enough to approach me directly for my number or dates?