If the prompt is as long and unique as other copyrightable writing (which includes short works like poems) then why shouldn’t it be copyrightable?
Okay, so the prompt can be that. But we’re talking about the output, no? My hello-world source code is copyrighted, but the output “hello world” on your machine isn’t really, no?
Because like 99% of the users don’t use more than a couple hundred megabytes.