So, theoretically, if it was just as easy to kill lots of people with a pressure cooker bomb as it is with guns, then you'd see just as many pressure cooker mass casualty events as you do mass casualty events with guns. But we don't. When someone wants to go out and kill a bunch of people, they almost always choose guns. That suggests that when people want to kill others they believe they have a better chance with guns than pressure cooker bombs.
Guns are very efficient tools for killing people. That's what they are designed for.. requires not a lot of training, and are readily and cheaply available.
Over a large population you can lower homicides by raising the costs of committing a homicide and by providing better alternatives to homicide. Part of raising the cost is making it more difficult to do so. There is some percentage of homicides that occur because it is relatively easy to do so with not enough barriers in the way. Get angry, buy a gun for very little money, and go shoot people.
So, sure, there are definitely cases where someone is determined enough to kill despite all obstacles, but over 300 million people and 25000 homicides, some percentage would not occur if it was harder to do so.
Right now for every person in the US there are 1.2 guns. That's twice the number of the next highest country (the Falkland Islands, and then Yemen). Basic economics - more supply = lower cost. Guns are crazy cheap and easy to get here.
However, we do have the 2nd Amendment, and the way it's interpreted means gun control is difficult, so it's not likely going to happen.