For the same reason Anya is an ex-demon and not a "demoness" and Buffy is a vampire hunter and not a "huntress." I believe it's because some gendered words don't have the same impact, they tend to drive attention more to the gender of the character than to their power. Like with Dawn always correcting Xander about being a "centaurette" instead of a centaur in season 8: he talked about her mythical nature, but she insisted she was still a girl. Or like when an interviewer asks someone about being a "female writer" or a "female comedian" etc. instead of about their job overall.