Lots of reasons. But I'd say the culture of glorifying gun ownership is a root cause combined with an institutionalized and deliberately flexible attitude to any form of gun control. It varies between states of course. But the net result is that people are assumed to be armed to the teeth. Which leads to shoot first and ask questions later situations. Which is actually something you can legally get away with under some circumstances and something that is glorified as well. And it's getting worse because it's getting worse. People respond to feeling insecure by getting more guns. And bigger guns. And this is big business in the US. Gun manufacturers advertise on TV, lobby the local authorities, and spend billions on marketing. There's more scrutiny on advertisements for cigarettes and alcohol in the US then there is on gun commercials. There are more rules around owning a drivers license or a car than there are on owning a gun.
That's why things have gotten much worse in the US in the past decades. The rules haven't changed but the number of guns has. And given the percentage of people with guns and the percentage of people with mental issues, the Venn diagram of those two groups intersecting produces a percentage of armed homicidal maniacs at a high enough frequencies that stuff happens. Simply put: when millions of assault rifles get sold, a few of them are going to end up being used by exactly the type of people you'd never want to own such a thing. Simple math. There's nothing mysterious about that.
I'd probably want to own a gun if I lived in the US. There are war zones that have less casualties per year than some cities and areas in the US. I think Kiiv probably has better statistics than Chicago in the last half year or so. That, or it's alarmingly close. Combat medics actually get trained in hospitals in in big cities in the US. If you work in the ER in Chicago, LA, etc., you are going to get plenty of practice. Every day. So, yes, I'd probably feel a need to defend myself in such a place. I'd be part of the problem.
However, I don't live in the US. I live in Berlin, Germany. I feel safe here and feel no need to own a gun and tough luck if that weren't the case because it's pretty hard to get one here. It's not a perfect city and there are occasional incidents involving gun violence just like everywhere else. Generally if you see a gun you are either in an armed robbery, looking at a person in uniform, or in the country side with people responsibly handling their legal guns while hunting. The first happens but not very often. Most crime does not involve guns. And when it does, the sentences are steep, the guns are typically small (easier to conceal), and highly illegal. Anything involving illegal guns is not cool with the authorities. People in uniform discharging their arms is generally involves lots of paper work and scrutiny. More so for civilians with permits and legally owned guns. Getting permits is not super easy and keeping your permit requires sticking to the rules. So, that narrows down the situations that can escalate considerably. It still happens but not very often. Same math, different variable values.