It's possible she's as shy as you are. It's possible she thinks since you don't say anything you aren't interested. It's possible she's just friendly and not really interested. The possibilities are endless as to why she doesn't. The biggest problem is you won't know if you don't ask.

IF you can't say hi can you wave at her the next time she looks at you? Just a little one to let her know you see her too. Would that be possible for you?