Why can't a guy just go up to a girl and ask her out?

I don't know, I don't find it hard at all, you just talk to the person see what happnes, usually you get the right vibe of things :)

Guys: Guys are, as you said, afraid of rejection. It's not just personal rejection though, there are many far-reaching consequences. For example, in most of the "does she like me" situations on here, the guy and the girl are friends first. When a man asks a woman out, not only is he setting himself up for personal rejection (which admittedly sucks, but most guys can deal with it) but he's also putting his friendship with the woman on the line. If he gets shot down, not only did he get rejected, he just lost a friend (I'm not saying they'll never speak again, it'll just never be the same). On top of that, men and women communicate so differently that we usually have no idea if we're correctly interpreting the signals you're sending us. In most cases, when a guy asks a girl out, the guy is flying completely blind, with no idea whether or not she sees him as a friend or perhaps something more. Trust me: it's scary as hell.

Girls: Though I'm no expert, most women I've talked to about stuff like this give the same answer: they don't want to seem too forward. Long ago, society decided that the burden of making the first move was on the man, so if a woman were to do it, she would come across as too aggressive or needy. In an effort to set the record straight here and now, allow me to say this: Ladies, this is NOT true. Guys, any guy, would be thrilled if a woman asked him out. Not only does it remove the "first move burden" from him, it shows him that the woman is self-confident, a trait most guys find very appealing.

Some guys are just really shy or don't know how to approach a woman and come out and say what's on their mind.