Part of it may be because they like adolescent boys, but probably more because it's just easier to create a dress to fit on a straight cylindrical figure. Curves are complicated to cut a dress for.
On the other side, the women who complain the most about the fashion industry not designing for "women with curves" are not what I would consider curvy. Sophia Loren in her prime was curvy. Raquel Welch was curvy. Most of the complaining women are just fat.