Why?
Because we, the gun-buying public, believe that there IS a difference between cartridge "A" and cartridge "B" and that we are skilled, smart, and erudite enough to fully realize that difference in some significant way.
We believe that there is SOMETHING that a .270 will do for us that a .30-'06 won't. Something that a WSSM gives us that a RUM or a plain ol' Magnum won't. Something that a 7-'08 will give us that a 7x57 won't. Something that a Bee will give us that a Hornet won't. 37 more ft.lbs. 106 more fps. These things matter!
We believe in the magic of numbers. 4,000 fps. 1,000 yds. Not just arbitrary quantities directly related to no fundamental function of marksmanship or accuracy -- these numbers speak to us and convince us that what we have isn't as good as what we could have. We must have MORE.
Or less. Who doesn't need a reduced-recoil tactical load for their Super-Short-Ultra-Magnum? Just makes sense, right?
Or we must strive for the MAXIMUM, absolutely superlative, most EXXXTREME level of perfect compromise. The best round is the one that pushes the highest velocity but fits in the shortest rifle. Or, that pushes the heaviest possible bullet out of the lightest, most compact autoloader. Or hits that magic middle ground between the quietest round you can put through a suppressor, and the round that tastes the best when holding the cartridge between your teeth.
While every game animal on our continent has been taken with any of a half-dozen or a dozen common cartridges, we know that there's something BETTER about the next "step up." If I've got a thirty caliber something, wouldn't I be better off with a .338? Or a .35? .375? Won't I be able to take game more ethically, farther, faster, deader -- if the number on my gun is bigger?
Or smaller! I could shoot little animals with a .223, but a .22-250 shoots farther. If a .220 Swift or .22-250 is just fine, and more than fine, then doesn't it makes sense to use something else? How about a .204? Or a .17" something? Why? Well, we can come up with reasons to explain every choice.
We don't want to use the same old cartridge that everyone else uses. We can have something special -- BETTER! Something that sets us apart from the crowd and says, "here's a guy who's not content with the status quo, but really knows a thing or two!" And, by the way, we also need commonality with others, and/or the "powers that be." So, we'd better have two -- our "Super-Short-Magnum-Lengthened-Improved," and our .308/.223 so we can always find ammo at any gas station, grocery store, or pet shop -- or take it off of the dead whomevers when they're piling up around our compound walls after the fall!
Mostly because we have no understanding of or ability to accept the idea of sufficiency or of diminishing returns. We have a dozen factors that might go into making a shot, and we can control maybe eight of them. Of those eight, probably six are factors of knowledge and skill developed through instruction, practice, and experience. The last two are "what gun" and "what cartridge." They contend for the bottom 2% of relevance, and drift ever farther into insignificance the more we nitpick between identical options.
So that's what we dedicate all our time and effort to. And the manufacturers see dollar signs every time they convince us that we'll be a rock star if we just had one more letter in the acronym of our cartridge name.
