Aside from a lot of guys being thickheaded and not seeing it, there's also selection bias.


A) A girl has a crush on a guy. He notices but plays it safe. Maybe she's just friendly. Result: no harm done except perpetuating the myth that guys don't notice.


B) A girl is just friendly but the guy thinks he's being crushed on and acts on it. Now he's forever labeled as a creep.


The only safe play as a guy is always, always assuming she's just being friendly. Unless she comes right out and says she wants to hump your bones, just assume she's being nice to you.