Because real men should never do anything that isn't completely self-serving.
I hold the door because it's polite. For women, for men, children, the elderly, even people I don't much like. I'm not trying to look good, I mean who the fuck am I impressing between myself and a single person I'm probably not going to see ever again and who isn't likely to go about telling everyone they know that one day a guy held a door open for a few seconds?
But then again, the idea of holding a door bears explaining to you people because everything you do actually is self-serving and you find it incomprehensible that other people don't inherently share the same motives, so you write it online for your fellow sociopaths and chuckle while thinking you're on to "the game" you think humanity is playing. And how losing is winning.