This is the stage we all state how we are shocked about the mass murder that has taken place, lots of public chest-beating but there will be no action by the government to put in place any solutions to deter any further occurrence. You raised a number of points as to the cause of these mass murders, the only one specific to the United States is “the lack of laws.” All other western countries have all the other issues, yet do not have a daily mass shooting. The NRA and the gun lobby are better organized and funded than any group trying to implement any type of control over guns, so do not expect any change. Comparing the number of deaths by guns from country to country is simple statistics and facts. We need to look at society as a whole, a much broader investigation is required. Why do we hate ourselves and each other so much?
Suicide rates (the preferred method being shooting oneself) and overdose deaths from opioids have caused a decline in overall life expectancy in the United States, again the only western country with a declining life expectancy.
It would be sensible to conduct studies into “why” we endorse mass murder, suicide and choose oblivion over life. However, again the NRA and special interest lobbying (gun and pharmaceutical manufactures) prevent government bodies such as the Centers for Disease Control and Prevention's (CDC) National Center for Injury Prevention and Control (NCIPC) from investigating the cause.
So, we are effectively prevented from understanding the “why.” Therefore, linked to the question “why” do we do it, should be “why” don’t we change?