Because some guys have manners. I do it for everyone, not just girls ^•^
Why do guys open doors for girls?
It's called courtesy.
About 5 years ago on a cross channel ferry, I was approaching one of the deck doors at the same time as a petite, pretty dark-haired lady. In a moment of gallantry, I stood aside and held the door open for her.
This was the cue for a slightly drunk nurk on the other side of the doorway to try to push through. I firmly pushed him back, waiting for the inevitable reaction from him (which never came).
"Thank you, Sir" came from the woman, together with a big smile.
She then turned to the guy I'd just shoved out of the way and and used a stream of foul-mouthed inventive that I'd never heard from the lips of a woman before. (It turned out she knew him). That entire side of the boat went silent.
I'm not sure that the story adds anything to the debate/Q&A here, but it was funny at the time.
I normally open doors for ladies, but it got me in trouble once. The lady was a feminist, and she says very loudly, "Don't do things like that women are quite capable of doing things for themselves"
Next afternoon, after work, I saw her struggling to change a flat tire in the parking lot. I said "Good afternoon" and went to my car. Got in trouble for that the next day at work. So damned if you do, and damned if you don't :)
Basically it is considered good manners and a courtesy. Personally I will hold a door open for almost anyone.