One obvious reason is that a boy is destined to support his parents and much of his family all his life, and therefore represents a long-term investment, whereas a girl will be given over to her husband’s family as soon as possible.
Una razón obvia es que un niño está destinado a sustentar a sus padres y gran parte de su familia toda la vida, y por lo tanto representa una inversión a largo plazo, mientras que una niña será entregada a la familia de su marido lo antes posible.