There could be a variety of reasons. Maybe the writers are influenced by traditional gender roles and don't try to break the mold. Or, they might focus more on plot and action, neglecting the development of female characters. Another factor could be a lack of research and input from women themselves.
Sometimes, it's because the industry standards or reader expectations push for certain tropes and archetypes. This can lead to one-dimensional portrayals of women. Also, a lack of diverse voices in the writing community can contribute to this issue.