This is a legitimate question. First, think about the sheer size of the atrocity. The number of victims claimed by gendercide exceeds the number of deaths in World War I and World War II combined.  Gendercide is the largest atrocity the world has seen, rivaled only by the genocide of indigenous people in the Americas -- a loss of life estimated between 55 - 100 million people.
Second, consider that when earthquakes, floods, or famines occur around the world, we generally rush to help. But the ongoing decimation of women doesn’t make sensational headlines, so we tend to ignore it. Gendercide is an atrocity much like slavery in the American South; at the time many believed slavery was wrong yet also believed nothing could be done about it. It was a small number of visionaries who convinced others that slavery could be eliminated. We do not need to go to war to stop gendercide. We can address it peacefully. But let’s not find ourselves on the wrong side of history by failing to act.
Third, we care because we do have the power to help some, if not all, women. A little education or a micro-loan can work wonders for a woman, enabling her to grab a tiny slice of the economic pie. Women who make money become more valued in their communities, and unlike many men, they use nearly all their earnings to provide for the health and well-being of their children.