Amazingly enough, there are people out there with iOS devices that Apple have long since kicked to the curb of obsoletion. Not everybody hurls their iOS device down a flight of stairs as soon as Apple decide its time for you to buy a new device for the latest iOS.
Pretty sure there are lots of relic files ("archives") on the internet. I bet if you look really hard you can find a version of Windows 95 floating around. My goodness.
Because they can. Maybe in the future they will only leave the latest version of each fw just to cover bases for older devices that people fix, but so far they don't have an issue with hard drive space to store these small files.