@icekatze They don't... you see, authors often add an extra girl into a series when there is already a main girl, and that extra girl is often a childhood friend, so that the author can add an extra love interest without much effort...
But because it happened so often, the myth that childhood friendship always fails came about when in reality, it's just that main girl is the one that'd normally win... being childhood friend actually means nothing in stories... it's your actual position (main girl, secondary girl, side character) that matters...