Because it wasn’t created by a human being.
If I ask an artist to create a work, the artist owns authorship of that work, no matter how long I spent discussing the particulars of the work with them. Hours? Days? Months? Doesn’t matter. They may choose to share or reassign some or all of the rights that go with that, but initial authorship resides with them. Why should that change if that discussion is happening not with an artist, but with an AI?
The only change is that, not being a human being, an AI cannot hold copyright. Which means a work created by an AI is not copyrightable. The prompter owns the prompt, not the final result.
How do the actions of the prompter differ from the actions of someone who commissions an artist to create a work of art?