I was always able to maintain female friendships as 100% platonic on my end (But I'm also admittedly low T in general) but it was always the girl who caused the friendship to breakup. Acting too weird or sensitive about stuff that I couldn't for the life of me understand, or just going out of her way to disrespect me as a person to make another man happy.
I think online friendships are easier because there's not that "physical" aspect of it, if the concern is attraction I mean, but most people are going to do the same thing, where they're going to prioritize their sexual and romantic relationships over your platonic one. I think same sex friendships are easier to maintain just because that person is going to be the most similar to you, and I think both genders are just so different in so many regards that's where the main conflict ends up happening- is what each gender usually ends up prioritizing.
It's the small stuff that builds up over time usually. While I'll always try to listen, a lot of the subjects are not going to be that interesting to me as a man as it might be for another girl. Certain games, talking about boys, makeup/nails, etc. As a man I don't really care that strongly for that stuff every time I talk to someone. In occasional doses it's not a big deal but y'know.