Same thing that makes them rape women and beat or murder their wives and girlfriends, I guess. 98% of mass shooters are men. A woman is raped every 2 minutes in America. Every minute of every day, 20 people are abused by an intimate partner with most of the abused being women. In 2020, 47,000 women were murdered by their intimate partners. Domestic violence help centers get 20,000 phone calls per day. What makes them do it? Maybe some toxic soup made of patriarchy and testosterone? I don't know. Maybe they should be the ones answering that question.