Allah, The Creator of male and female, The All Knowing, The Most Merciful, The Most Wise, knows when girl becomes Baligh and when boy becomes Baligh. Our knowledge is very limited and we can not challenge His Knowledge, Wisdom and Mercy.
Girl becomes Baligh when she completes nine lunar years much before the age of a boy when he becomes Baligh. The wisdom behind it is best known to Allah, the Prophet (SAWA) and Ahlul Bayt (SAWA). The intellectual abilities of the girl become perfect when she completes nine lunar years, but boy needs more time to reach to that intellectual abilities. Baligh girl is not a child even if western society calls her a child. She think and weigh things in a different way if kids thinking. This might be one reason,but we always believe the Allah (SWT) knows the best.