Videos from today's mass shooting at a Mad Butcher grocery store in Fordyce, Arkansas. At least 3 people were killed and 11 people were injured, including the shooter.

As much as everyone outside of America think every single person is carrying a gun at any given time, it's pretty uncommon. Even in places where it's very easy to carry from a legal standpoint it's still fairly uncommon. Most people generally just don't feel comfortable enough with a gun to actually carry one, and the ones that are comfortable with firearms don't necessarily want to deal with the legal and social aftermath of having to use a gun to defend yourself. Just because you have the ability to actually carry the gun on you doesn't mean there won't be massive legal fallout for yourself if you ever actually had to use it.

That being said, low-end estimates for the United States are in the range of 55,000 to 80,000 incidents per year, while high end estimates reach 4.7 million per year according to Wikipedia. It's difficult to get accurate data on such things since many defensive gun use incidents aren't reported and also because the political nature of such data and people on both sides of the argument wanting to distort it to their benefit.

Despite having the right to do so, there's still a social stigma against it. A lot of people have a high degree of faith that the police are sufficient protection. A lot of people feel that owning a gun makes them less safe, like they'll have an accident or something. Concealed carry is not legal in every jurisdiction, and for that matter some places outlaw handguns, making it difficult to legally carry in such a situation. It's also hard to effectively conceal carry when you're as obese as a lot of Americans are.

But really I think it just boils down to cowardice. They know if they stand up to take a shot, they make themselves more of a target, so they cower and hope someone else will deal with it.

But I say that as someone who's never been in this situation. I honestly don't know how I'd react.

You need a CCW to carry a firearm on you in Arkansas

It actually happens a lot, but many news media organizations don't cover it either because it's either counter to their agenda or because its not sensational. A decently sized chunk of the population of the US does carry, but not enough to stop every event.