While the Chinese mind functions many times in what we westeners may think is totally not logical, for them is normal.  Take two girls walking on the street holding hands, for us they could be lesbos or similar, for them it is as common as rice with meals, and has no other meaning except that they are friends.
Clathes.  For Westeners, there are men (or boys) type clothingand women (girls) type clothing.  For Chinese, they are just clothes to be war if you fancy them.  Take bikes, in Western countries you have bikes for men different than bikes for women.  Hardly no one will use the "wrong one" unless a dire emergency was up.  Here, they are just bikes to be used, and they will buy the cheapest, with a horizontal bar on frame (for men) or not (for women).

Have you tried waist lenght hair in summer in China ?,  Mybe the heat and sweat will dictate for some Ladies to be incluned to a shorter haircut.  Also easier to manage, wash, comb, etc.  In other words, more practical in their eyes.  Why worry as to the reason for it, just accept it and worry about more important things, like not getting hit by a car while crossing an intersection.

Because they like it. Duh? It may seem a little stranger to you because guys and girls here are a tad more androgynous, but how is it any different in Western countries? People wear what they want. They cut their hair the way they want. Just because you don't like it, doesn't mean they can't like it.

It could be because of the Chinese  media.  The Korean/Chinese TV soap operas got something to do with this. From what I was told, the story line is usually about a boyish girl finding true love and such. Kinda like 'Mulan' style. 

Or maybe because short hair is a wash n wear style. 

Sorry dude, I love the tom boys. Always have. Nothing is sexier than a woman who doesn't mind killing spiders for me.