It is much easier said than done to defy gender expectations that have permeated society for centuries. Women who are raised within a patriarchal society are conditioned since childhood to believe certain things regarding gender, and shaking off those viewpoints is incredibly difficult and takes a long time. In addition, refusing to conform to certain expectations, such as the expectation to marry and have children, could result in serious strife within a woman's family (especially when the woman's parents desperately want grandchildren). It is sometimes easier for women to do what the patriarchy wants them to do.
It can also still be considered gender discrimination because these expectations for women exist due to the patriarchy, which is upheld by men in society who make no effort to acknowledge it, question it, or make any attempts at dismantling it.