Serious question: Why are so many people "going postal" in the US as of recently? "

Mental illness has nothing to do with it. The overarching problem is right in front of everyone: how the media covers mass shootings. But people aren't smart enough to recognize it and instead waste time debating non-solutions like stricter gun control laws or better mental health care. Because of that, nothing is going to change. American society simply isn't astute.