Why do you think women keep their hair longer than men (in most cultures?)

Mainly because long hair is a conventional beauty standard, so even the plainest woman holds some leverage if she has "pretty" hair aka long hair. But it's also a lot more trips to a hair salon/barber if you have short hair, and it costs women more to get a haircut. Why spend more money to look worse in the eyes of the public? All that said, I cut my hair short last year and love it, though it does mean a lot more trips to the salon.

Women grow their hair long because they can't grow beards.

Long hair is considered sexy and feminine. Question is how did that idea start?

Despite their protests otherwise, women (most, not all) do whatever they think will bring them the most attention from men. Men generally prefer long hair on ladies, so that drives the tradition. Long, healthy hair is also one of the visual cues to men that a woman is fertile. At the end of the day, it's all about catching a mate.

A lot of those points but remember, women used to wear their hair up for centuries except for the bedroom.

In the roaring 20's, flappers cut their hair into a bob, dropped the waistline and stopped wearing corsets.

30's to 50's saw women having shorter hairstyles. That was the fashion, and the daring women of the 20's paved the way for that.

60's, women had longer hair, beehive styles.

70's, was the influence of the 60's hippie, so many with long straight hair parted in the middle. I notice according to my son's yearbook, that is the style today among a lot of teen girls.

I like mine long enough to be able to put it in a ponytail while at home, but I take it down when I leave the house, probably because I think I look better with it framing my face. However, most days now I don't bother to wear makeup in or out so go figure.

I think men are more attracted to long flowing hair, you can't say a beautiful head of hair isn't impressive.

Long hair on a woman looks great.