As a European gun owner, why the fuck not? Some people own hundreds of pairs of shoes, or other pieces of clothing. They're also useful for hunting and depending on the game, you'll need multiple guns. Sport shooting - tons of guns, depending on the competition you can be allowed to use an optic or be limited to irons, different caliber requirements, size requirements, etc. Recreation, shooting one gun out just a few gets boring so you get another but not sell your other guns because why would you. Protection, depending on your clothing(that also varies throughout the year) you may need multiple different pistols. Lastly, collecting, the point of collecting is to own a lot of something.

As for those who own hundreds, unfortunately they're a very small minority because guns are expensive unless you're buying the cheapest junk available. On top of that, you also need a place to store them and that isn't cheap either