@nikov,
Because you have invested far too much into this fantasy.
It's a fantasy until you open your mouth and start talking. So start talking. It does not have to be anything deep, and for God's sake, don't talk about feelings. Say hello. Ask about work or school or whatever applies. Keep it light and make it last less than 5 minutes.
Do this in person, not on Facebook or any other form of social media.
And by the way, you have no idea how she feels, seeing as you can't even say three words to her. You could be misinterpreting everything here. Finally, why didn't she friend you on social media? Because you are not friends. Right now, you are just some guy who stares at her.
Start to talk to her, even a little bit, or you will always just be this random guy who stares at her.