Why is it polite to do things for girls? Or let them go first?

Like open doors for them, ladies first and other things like that? I don't get it. Grownups always tell me to do things like that but I don't understand why. I mean I'm pretty sure they can open doors themselves. I think its weird in my opinion. I would only help a person if they need it not because they are girls. Why do people say its polite? Also why isnt considered as polite when a girl helps a boy?