Because people purchase it.
It is a trend. Younger kids want to look like the older kids they see looking "cool" (in their own minds)and skulls have been around for years. Surfers, skate boarders, Graffiti..
Pirates of the Caribbean is when it started way back when, but like I always taught our daughter. "If it is in fashion at Target and Wal Mart, it is old hat for people really into fashion." These companies are not trend setters, they are trend followers. Usually 2 years late.
I agree to purchase as many basics as possible. Kids out grow and wear out clothing so fast, if they do have one skull shirt to wear in October, they will out grow it during the Christmas holidays anyway.
There are some young girls that like the "shiny fashions" Heck check out the adult females.. Shiny shoes (yes I am guilty), tops, sweaters etc.
They will always be around, but if our daughter wanted something like that Or was given a gift like that, I might purchase a few things, but I told her those were not "school clothes"....They were "weekend clothes".
Shiny clothes for shiny personalities! Ha!