I haven't been girly for years but it's not as if I look at men and think that I want to dress like them. I just want to be comfortable.
Back in the day, I did wear colorful outfits, skirts by choice, and play with makeup and nail polish. This was all before the age of 13 or so. Then I started to dress more "comfortably." This coincided with the grunge era so a lot of girls wore mannish clothing anyway.
Other factors for me, personally, are that I've experienced negative male attention due to my body and don't like to draw attention to myself. That and I have skin sensitivities: I'm allergic to most makeup, perfumes, soaps and shampoos. I also could never get a handle on earrings. (I don't know if it has to do with allergies again or sensitive hearing -- I just always found earrings annoying when I had them.)
Clothes shopping is always a demoralizing experience because I'm 5'3" and have G cups so hardly anything ever fits my upper body properly. Sometimes I have to shop in the junior boy's or men's section to find something to fit my upper body. Women's tops are either far too wide at the shoulders with awkwardly wide necklines or they stretch awkwardly over my boobs (if they even fit at all).
I loved living in California because I could get away being casual more than I could on the East Coast of the 'States. I often wore shorts (sometimes even short shorts) and T shirts or tank tops. Unfortunately, I'm not young nor am I fit enough to get away with any of that anymore. Shoes, except on special occasions, have always been flat, whether sneakers or sandals.
Having to go to events where I have to buy a new dress, with matching shoes, and matching shawl and matching purse with matching jewelry, matching hairstyle, and nice make-up drive me up a wall.