In America it is work or starve for most people, (and work ain't easy to come by right now, 10% unemployement rate if your lucky in most areas). But yeah, jeans or slacks are more suited to equal oppurtunity jobs, they are utilitarian wear and skirts aren't. Still, there are jobs in the USA that still require skirts, just not that many.
Do you have a lot of Japanese woman dockworkers, forklift drivers, construction workers? I bet female Japanese chefs would not be caught dead in a skirt while they are working. There was a lot of crap that came with womens lib but it sure helped guarantee equal oppurtunities for women in work places that were denied them before. Pants are just more practical in most cases. ( I cant see a girl driving a front-end loader in a skirt)
Have you actually seen any JAV? short on good size tits many of those girls are not. Besides skirts have nothing to do with a girl being able to show off her boobs and looking at boobs is a male pastime no matter where you are from. ("Eyes up here please" must be commonly said in every language).
Trust me, women have an idea of what is attractive, it is part of who they are. Women develop this unbelievable peripheal vision so they can see if guys are looking at them without anyone noticing. Plenty of American women wear skirts, (and dresses), but they do it when they want someone to notice their legs. Doesn't anyone here actually go to clubs? (I dont much anymore myself, too old for most of them). Has anyone ever heard of cheerleaders? not many pants on those gals. There is a huge diversity of womenwear in America and the women themselves are very diverse in all forms. So you arent going to see a lot of school girls in skirts unless it is a catholic or maybe a private school that requires them as part of the school uniform, (and there are thousands of schools in America that do). According to Census 2000, 281.4 million people were counted in the United States - 143.4 million of whom were female, (more than twice the amount of Japanese females and more than the entire population of Japan as of 2010). Making any general statement about that many people is just not a winning choice.
So why do Japanese women wear skirts? There are many good answers to that already in this thread...pick one.
...and again, another good answer from this guy here! you should read Ceewan's