St.Valentine day is famous in Japan and other countries. White day is only Japanese event. March 14 is a special day for men. The men who were given chocolates should send candies for thanks of Valentine day. White day is not big event comparing to St.Valentine day. It is clearly commercial event for Candy makers. The men who were given Giri choco often say " I don't want to get chocolates on Valentine day because it is a little troublesome to think what candies I should give."
And women don't expect the candies on white day all that.
Japanese like to set special day for business profit. Anyway, I think that white day isn't necessary.