I think it's because most projects don't have any settled place (or at least, any place that lives inside the project's source-code repo) to record PRD-like information.

But because AI coding agents really do benefit from having PRD-like info (i.e. a hierarchical breakdown of the "intended what" and "intended how" of the system that "should" exist in the repo, kept updated to reflect changing ideas about those intentions), they really want to record that type of info somewhere, so they can then reference it.

AI coding agents don't seem to have any "taste" (or maybe "confidence"?) when it comes to asserting that even if you don't already have a separate PRD-like file in the repo, you should have one, if you want the agent to do its job well, and so it would request you to let it create one (or just automatically do it.) Instead, they just shrug and cram the PRD stuff into whatever structure you do already have. Which, if you're starting fresh with a greenfield project, is usually just a README.

(I hypothesize that if you tell an AI coding agent to use Behavior-Driven Development [BDD] on a greenfield project, it'll probably lean toward maintaining its PRD-like info in the form of BDD test cases. Haven't tried this experiment yet; would be interested to know the outcome!)