it can be a multitude of reasons for the individual.. it can be manifestations of confidence, or insecurity, of political views, of pratical considerations, etc. etc. the list could go on.
The overall reason though is simple: fashion + culture
In China its 30-35 degrees every day where I live, but you will pratically never see any girl with any part of their chest showing. Legs are ok, but no chest - and almost no belly/mid showing. Its so rare for my wife to see a single girl showing any chest at all, that the last time it happened my wife pointed it out in shock to me, as she was so surprised.
When we are back in England its just common place in the summer though. Are English girls more insecure? More desperate to look good? etc. - I would say no. Its just a difference in cultural attitudes and a difference in fashion trends between the two countries.