America's greatest export is war. we're a very violent nation. every generation since the founding has had a major conflict to attend - but most of our wars in the last 30 years or so havent involved a lot of "boots on the ground". that pent up rage requires an outlet. mass killings/shootings/etc are a direct result.
adding to that, handguns are very easy to obtain - you can legally purchase them easily enough but there's also a very active black market where cash is king and questions are not asked, backgrounds are not checked. since Americans are legally permitted to own/carry firearms (as enshrined in our founding governmental documents), the system cannot change. it's not politically acceptable to request a change. no career politician is going to burn their entire life's work in something that is destined to fail. even considering is is borderline insane.
dont expect things to magically get better.
yes, but we closed all insane asylums/mental wards too. some hospitals have a few beds dedicated for that but it's not the massive infrastructure of the past.