Maybe the game starts on laundry day, and they were out of clean leggings?
Given the armour design for female characters in some games, I barely noticed the lack of leggings. Concept art for the female character in Original Sin shows armour which is much less functional (see
here, for example), though fortunately in the game you will not be restricted to that design. Actually, I don't know if that design is even in the game; it works 'better' in a fair sized concept art than it would when the character is an inch high.