Why do good guys usually end up being friend zoned?

I have seen this many times that a good guy helps his female friend everytime, but always gets friendzoned. They confess their feelings but they aren't accepted as boyfriends. What is it that leads them to being friend zoned eventually.
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If a guy has most of good qualities, like being sweet, understanding and always has your back, then why are they always ending up in friend zone. If I am not wrong, these are some of the qualities women want in a man. I get it if a woman already has a specific preference for men, but what about those women who are trying to find good men with the respective qualities but friend zone the very men with the same qualities that are around them.
Why do good guys usually end up being friend zoned?
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