I think you're defining them as "nice" because they are mostly regular guys and you think how they engage with you is how they engage with everyone.

This is a false assumption.

You think they get rejected despite being nice guys because they're awkward or not conventionally attractive.

But being "nice" doesn't make you kind or decent or honorable or trustworthy or genuine. It basically means you know how to be polite when it suits or benefits you. It's a veneer.