I’ve just re-read this post and it’s probably a little to frank and honest for its own good. Please consider that when I’m talked about how shallow men are, I more mean young twenty-year-old men. We do get a little better, as we get older.
My take on this subject is slightly different to every answer, with the exception of P-puy.
Firstly, I think it is important to point out that men don't have one girl they like, but more of a shopping list of girls. We always have at least a hand full of lasses that we wouldn’t mind asking out at any one time. There maybe girls higher up that list than other, but it is always a list and not just one girl.
Secondly, men are scum. (You probably already knew that.) At the start of a relationship when your first dating, a large part (not completely but a large part) of how we measure the success of a relationship is what we physically getting. i.e. 5 dates and all you've done is hold hands, you can guarantee there won't be a 6th date. Sorry to say it, but we fellas are that shallow.
So my observation is.
The boys don’t exclusively like you; they like you and other girls at the same time. So your statement "I don't understand why they openly flirt and date other girls, but secretly like me." is a misinterpretation. They do like you and the other girl’s their flirting/dating. I'm assuming as the guys have told friends they think you’re cute and sexy, then you probably are.
SO THE MEN LIKE YOU BECAUSE YOU'RE SEXY, AND NOT BECAUSE YOUR SHY. HOWEVER THEY ARE NOT ASKING YOU OUT BECUASE YOUR SHY.
why don't they just admit it to me instead of my friends?
Because you’re shy, you’re a high-risk date. i.e. They will ask you out, go on a few dates, spend a lot of time having social awkward conversations and spend a lot money (and your a girl, so you probably never thought about this, but dating is very expensive for men.) and will get no physical satisfaction. And they’re thinking this because normally shy quiet girls are normally shy and quiet in the bedroom/on the sofa. I’m sorry but us men are that shallow.
There are exceptions like that filthy Willow from Buffy the Vampire Slayer, but she is an exception.
I have given the following advice on this web site before and been heavily critised for it, but that won't stop me giving the same advice again. I'm European; I'm assuming your American, so this advice my not travel across the Atlantic.
Its 2008, if your a girl and you like a guy, their is nothing wrong with you asking him out. My current girls friend of 8 years picked me up in a bar.
If you’re too shy to ask a guy out, do the other thing women do and just make your self blatantly available. "Oh, you want to see the latest Batman movie? I really want to see that too. I have a free evening on Thursday."