It makes total sense to me. A phone case is just a cheap piece of plastic that's made using cheap mold-manufacturing, and cases last for the entire life of the phone, sometimes even living on through a couple of phone lifetimes.

But screen protectors have to be more rigorously designed, - making sure that the material works well between finger and each particular touchscreen, and it's made using relatively much more expensive manufacturing processes like curved glass cutting, and people have to replace them every once in awhile because the purpose of a screen protector is to take all the damage that otherwise would've happened to your screen.

Screen protectors need to be changed way more often than a good case and they are usually similar is prices so this makes sense to me.