Because prototyping requires you to build and then get feedback, because almost no company knows how to actually plan a project in isolation rather than have to

Because prototyping requires you to build and then get feedback, because almost no company knows how to actually plan a project in isolation rather than have to go through multiple iterations. Then rework (where you're redoing things based on user feedback and changing requirements) causes forward progress to not be the sustained motion that management always expects. That's why.