Because there are just so many interesting ways to all do one thing...send a piece(s) of lead(copper, steel, etc) down a tube to strike a target.
The history of the firearm is a history of mechanical engineering. It is a history of the industrial revolution. It is a history of defense. It is a history of human survival.
Firearms are also a record of many cultures from around the world that display in full glory the arts and sciences of those cultures. From the most crude of early Phillipine rifles to the most ornate silver inlaid Russian shotguns to the intricately damascened Turkish rifles or the delicately engraved rifles of England. All made with the most exotic and finest materials made at the time. Pearls, silver, gold, coral, ivory, ebony, ruby and much more have all ended up on firearms.
Beyond the history and aesthetics there are some very practical purposes for owning many different firearms. Foremost is finding the caliber and platform that work best for you. What works for me or anyone else here or elsewhere on the web may not work for you. I'm very finicky about what firearms are best suited to me. Some folks like some flavor of 9mm or .38. I prefer something larger. However, I would most likely have not reached that conclusion had I not shot thousands of rounds of 9mm/.38/.357 through bunches of different firearms both wheel guns and autoloaders. I prefer .40S&W/10mm/.45ACP. And these I have shot on many different platforms with many different loads. Right now for defensive purposes the 10mm has just a slight edge over the .45. However, this is my choice. This my current judgment and what works best for me according to my criteria. I wouldn't dare suggest or say that these will work best for you.
For longarms it's basically the same process. I have settled on a 7.62x51 Nato in an FAL platform as my defensive longarm. For a pump action shotgun an Ithaca 37 is my choice. For sporting purposes, anything goes. BB, pellets, crossbow, bolt action, break action, auto-loaders in any caliber and configuration. I don't hunt but, I do slaughter pop cans, plastic bottles and jugs and I am ever at war with paper. I'm not some big time shooter or competitive shooter. I only shoot between 5K and 10K rounds/ year. I probably spend more time admiring and caressing my guns than I do shooting them. And yes. I have hugged my guns today.
Chipper