People value men based on self-confidence we project. The thing is, a man without self-confidence isn't perceived as weak. He's perceived as scary.
Statistically and physiologically, men are a gender of anger and violence, much more so than women (who are dangerous in more subtle ways). If you lack self-confidence, it sends a signal that you don't know what to do with your raw power. You're prone to misuse it, you're prone to act out, you're prone to do something dangerous - hurt someone or yourself, potentially - because you aren't at ease with yourself.
A confident man is attractive because he projects safety. Not just safety from others, or safety from external circumstance; but safety from his own whims and impulses. If he's self-confident, it means he's more likely to respond appropriately to situations. It means he's less likely to lash out; less likely to be a source of harm.
Being attractive, as a man, is largely about assuring a woman that you aren't crazy and dangerous.