Because men don't get penalized for doing it and they feel completely entitled to keep doing so because of cultural reinforcement - despite it being foolishness - and being annoying as hell. That's why it's described as part of privilege.

I'm a research-based writer. Nearly everything I put forth is filled with data, citations, quotes from experts, etc. And yet every single time, some guy (at least one) feels no compunction at all about telling me that I'm wrong, that I'm uneducated, that I'm naive, or whatever even though they have no data, no quotes, no citations or anything else to support these assertions. In their minds, they think that is perfectly reasonable. After all, I'm just a woman. What could I possibly know (despite quotes from Scientific American, et al)? That comes out of male privilege.

I do value truth and charity - which is why I won't pretend like a very pervasive cultural dynamic isn't taking place - when I see and experience it on a pretty much daily basis. I don't accept bullying any longer - I've experienced too much of it. In my opinion, the best thing to do to help bring about a better world for everyone is to stop turning a blind eye to this sort of bullying, domination-based behavior.