Why would woman keep a friend if she knows he has feeling for her?

I have this woman "friend" in class, which I got a crush from the start. But not being that scared, I actually tried to interact with her more (we were in the same group for work, during breaks I went to talk to her and her friend, we even spent some time outside of class). I know I've been stupid and I waited for some miracle to happen so I never said I really like her, I only told to her other friend. One time we were drinking in a group, and when she was alone she told me her friend told her that I like her, if it's true so I said yes, but things got complicated so I never got her reaction. After that she stayed as a friend, had dates etc.

After half a year when I went with her to her doors and a little conflict (she was talking shit behind my back to her "ex" she actually never seen, theyve been only texting, but he told me so I stopped talking with her for like 2 months), we started being "friends" again, but this time when I found out she has a bf I told her I had feelings for her all the time and we can't be friends. She was like it's allright, she will give me space so I can lose feelings. That space was less than 2 weeks when she asked for school notes (it's alright) and then started texting me again. On the school trip she and her friend even called me to their room, visited me few times and just did what friends do (I am also good friend with her friend, except I don't really have feelings for her). I would understand this because of the 2nd friend wanting to see me, but it was from HER initiation.

Now during summer break, one time she texted me how am I (we did a smalltalk) and 2nd time she sent me paragraph of how she's sorry if she dissappointed me as a friend and she appreciates me (all this time taken BTW). So why would a woman want to keep a friend this much if she's taken and knows I have a crush on her? Am I in the wrong because I should remind her more often I have a crush on her, or interact less with her? (I don't really talk to her now, most of the times she is one reaching to me).

Oh, and I think for a short time she liked me more, because I cut my long hair into short haircut, one time she said I'm smash now, she acted weirdly (asked me to go to the shop with her and then ignored me in the locker room, and was acting weird, my friend told me she prolly likes me if she does that), but at that time I knew info about her gossipping so I sent her a letter by valentine post (school thing), where I told her how I am disappointed and mad at her, where she reacted by telling me she still likes me and still will, she knows how she behaves and she had smth prepared for me too but is glad she didn't give it to me and it's psycho sending valentines like that.

So what should I think about it? And why would women want to keep a friend who told them he had a crush on her?