More to the point, many humans write in that style, which is where GPT picked it up in the first place.

Yea, it’s literally the style that is held up as good writing in the usual American high school setting, at least IME.

Yep, GPT's default "mode" seems to be the bland "five paragraph essay" encouraged by high-school courses. First, it tells you what it's about to describe. Then it describes it, inventing some supporting evidence. Then it restates what it just told you.

Sadly many students never move beyond this introductory framework, so their professional writing (which GPT trained on) ends up with the same uninspired style. It's a way to automate the creation of content that even the "lowest common denominator" of students can manage, but it doesn't produce good writing.