I have no idea what you've been reading/watching, but it's a meme that childhood friend NEVER wins. Its faaaaar from "most".
The only times childhood friends win are when it's a straightforward romance and there's no actual competitors, when EVERYONE is a childhood friend (Nisekoi) or when the new girl is also secretly a childhood friend (left, came back looking/acting different).
Just Google 'why does childhood friend always lose' and you'll get a ton of discussion about it.
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There was even a show that aired last year called "The Romcom Where The Childhood Friend Won't Lose!"
In fact, childhood friend winning is far more in a thing in western and American works (though western stuff doesn't do harem so there's usually no competition).
Childhood friends winning an actual romance competition in Japanese stuff is by far the exception.