This is obviously a very sensitive subject in light of the recent tragedy in Las Vegas, but I will try to answer the question.
Shooting probably satisfies some primal instinct to blast stuff. Can be a lot of fun too. Taken further, even if you don’t use a firearm for its intended purpose, there is the challenge of doing it proficiently and safely. Competive shooting is challenging and can be rewarding provided one has a competant coach or mentor and willing to put in enough range time.
On a personal level, dad flew fighters, and since the USAF theorized that clay pigeon shooting satisfied both skill practice and recreation needs, there was a skeet range at every base we were stationed. My brothers and I worked at them in high school on weekends and during Summer break.
Then there is hunting. In the Southeast USA where I grew up, it was sort of expected that you became proficient shooting quail, dove, and squirrel before you got pushed out of the nest. I had my first side-by-side 20 GA at 11 years old. Still have it. I would imagine boys from rural OH, PA, NY, or any number of States had the same experiences.
As Americans, we probably all imagine ourselves as just a few generations separated from Meriweather Lewis, Daniel Boone, or Jim Bridger. For better or worse, firearms are woven into the fabric of our origin and existence. Add 9/11, a strong firearms lobby, a bunch of Internet forums, YouTube, and very tolerant firearms laws in most states, you have a perfect storm for widespread firearm and ammo ownership.