Why stay in the friend zone? I just don't get it.

OK so I have a friend and she has been hurt so many times but yet she comes to me and saying, I'm the best friend as I listen, I told her that I would date her, but she said "No. She just likes me as a friend." It's much easier to walk away and have no friends at all but that would make me selfish, but I am tired of this, keeps hurting because of her boyfriends, when it's good she leaves me alone, when it's bad she can't leave me alone. I just feel used and it hurts me.