It's the same in China too and it's simple to me.
People generally want to look good and look attractive to the opposite sex, if possible.
In eastern countries the vast majority of girls (much more than in the west), can pull off and look good in a short skirt or shorts because they are slender and generally have pretty nice legs. If you can get away with it why not?
As someone else mentioned earlier, girls in the west show alot more cleavage than asian girls do because that's what they've got and it what makes them look/feel attractive.







