People still find simple answers to the motives behind mass-killings, even if they are wrong and tangential at best.
Drugs, video games, anime, moral decay, atheism, etc.
Mental problems are always assumed but never challenged, because of their inherent complexity. The person who committed the killings was wrong in the head, end of discussion.
A mass killing is only completely senseless to a few. Most people wish to make sense of the world, and many powerful institutions, from politicians to the media, profit from providing a framework for understanding.