Why always I always become bestfriend, when I wanna be there boyfriend? So tell me what I did wrong when we were all good?

So I have been talking with girls online and through time I found something disturbing that is I always get best friend zone, ya you got me correct. Not friend zone but I become a best friend. So lemme get you a clear picture. since I want you to help me.
1st gir: We start talking as Stanger, texting basically. But since she was good at responding unlike some girls who don't show many initiatives at first, she was lively in talk and we start building nice talk day by day. We used to share lots of gif, jokes, and what most I always hey her Laugh since she likes my sense of humor. Tho there was less flirting, and yea we once had an ERP (it was very at the beginning, coz she might had thought that maybe just for fun and after this, we were done) but after that were still normal and yea she didn't allow me or brought this sexy scenes and talk she refrain from talking about it. Maybe not much okay. So yea. We kept talking for 3 months. While she had a boyfriend wasn't giving him much time so I felt I might have a chance. But at the same time when I started talking there was another who was older than her also talking there was one difference, he always being cute, soft, and sweet with her like yk all commenting, saying and all while I kept this quiet more of friends talk... so once when we were having flirt talk I said "Yea u like me I know" so she said like dude yk as a friend. But you are the best one. And she also told me about that guy. But at times she said we can be besties (I agreed coz we were having a nice talk and we were understanding each other While with thought might things also get close and romantic Later) so yea from then she used to share everything from SS to Chats everything. We had an everyday talk, she texts me every day, every night, and spam me when I wasn't available about "Missing me" 1/10 Times 🙂 yea that's a sarcastic smile. So yeah when she broke with her boyfriend, she starts dating that guy. And told me this guy was so good to her.
Why always I always become bestfriend, when I wanna be there boyfriend? So tell me what I did wrong when we were all good?
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