It does defeat the purpose of the shorts in that yours legs will only be slightly more ventilated than if you were wearing full-length pants, but I think it's more of a fashion choice than anything else.
I guess having such long socks might accentuate the length of the legs as a whole, and make them lock longer and more slender.
Perhaps there's something of the "school uniform" element to the look as well?
Slightly different from what you probably meant in your question, but girls aren't the only ones to combine shorts with longer socks...
I'm not sure where the trend started, but I remember being into the nu-metal/punk scene during high-school, and that was a trademark of several bands.
For example, here's a picture of Chino Moreno, lead singer of Deftones with his socks riding relatively high:
