There are many good reasons to care. Off the top of my head:
* Having a diverse work force makes it less likely that you will be blind to problems caused by unintended side effects of your product that only affect certain groups.
* Having no women (or people of color, or people with disabilities, or...) on the workforce, will make you less attractive as an employer to talented people in these groups. Ultimately, you get to hire from a larger talent pool, if you make an effort to cultivate diversity.
* Making an effort to attract a diversity of people will make you uncover barriers of entry, that you might not have been aware of, and removing those has the potential to benefit everyone.
And finally, even if you believe strongly that your hiring practices are not the cause of the lack of diversity, and that the real problem is elsewhere (in the education system for instance), you can still choose to be part of the solution by providing proof that there is a desirable future to be had for a woman who chooses to go in to a STEM field.