In the book Gathering Blue, why do Vandara and the other women think Kira is useless?

Here is a quote from the text:

"We have all tolerated her presence for these years," Vandara went on. "But she has not contributed. She cannot dig or plant or weed, or even tend the domestic beasts the way other girls her age do. She drags that dead leg around like a useless burden. She is slow, and she eats a lot."