Because in every economy we've looked at, gender income parity is strongly correlated with overall economic growth. In addition, research on microlending suggests that this link is statistically co-causal (improving either one improves the other).

If you prefer a more intuitive take, most of the attempts to dismiss the gender wage gap say things like "women take more time off for families" and "women choose lower-paying occupational fields". But there's no a priori reason those activities and fields should not be more remunerative than they are, and there is evidence that making them more remunerative is beneficial to the economy as a whole.