Why do some women get angry when a man who they friend zoned, chooses to move on and find a girlfriend?

I would like to preface this with, Yes i know i did most of this to my self, and i was even more stupid for letting it carry on for 5 years.

There is a coworker i was interested in for years. she's 29 now, athletic, pretty, and so on. Years ago i told her i was interested in her, and she said "Lets just be friends" Okay, i can live with that. So for 5 years i was there for her, When her ex cheated on her, i was there with ice cream and a shoulder. She made me promise that i would always be there for her. When another ex beat her to pieces, i helped her get away. Any time she had issues i came to her rescue. We even had weekly things we did, like Sunday brunch and movie nights. Any time she was with someone, i was her shoulder to cry on. any time she broke up i was yet again her shoulder to cry on.

The final straw was about 8 months ago when i helped her move, and i was putting together a TV stand in another room when i over heard her say to her friends "He is my person, my best friend, besides its not like any woman will ever want him, so i can just keep him around." That hurt like hell. Her friends didn't exactly react to it very well either. They told her that she was using me, and she responded with "you're just jealous you dont have someone like him in your life."

I started to put distance between her and i. One of her best female friends actually sought me out and talked to me. It turns out she's a physical therapist, which is cool. We also had a lot in common honestly we hit it off. We decided to start dating. I started to further distance my self from my friend. It came to a head when she demanded that i meet with her. Fine, we met. she asked me what was happening, i said that i met someone and im just done being used. She was PISSED. Said i betrayed her. Asked me who it was. When i said her name, she got even more angry.

haven't spoken to her in months now, Girlfriend, now fiance dropped her as well.

Updates
3 mo
I will add that said ex friend has tried repeatedly to sabotage the two of us. It got to a point where my girlfriend felt the need to stand up for me. Since i have been with her though i have gotten my life on track, lost weight, gotten more fit. and so on. The reason she felt the need to stand up for me is on our last meeting with the ex friend, she all but threw her self at me.
Why do some women get angry when a man who they friend zoned, chooses to move on and find a girlfriend?
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