I wouldn't say they have "underperformed", the best women chess players would absolutely wipe the floor with me and other casual players. However, historically women don't reach quite the same top level as men in some mathematical tasks, even though their median performance is on par or even better sometimes. You can visualise it as two normal distributions with the same mode, but one is slightly wider.

For now the WGM title exists, because if it didn't, there would be no women Grandmasters. While you see it as a consolation prise, I see it as an encouraging step to build a tradition of unisex chess participation so that one day we can have truly equal tournaments.