Another commenter above this proposed a pretty compelling theory for the source of this style: SEO-inflated prose online. If the models were trained on the internet, "higher quality" content needed to be indicated to them during RL somehow. Search engine ranking is an easy-to-obtain metric that's kind of like "quality" if you squint, turn around, and lobotomize yourself. So the AIs have a high likelihood of producing the kinds of content that is rewarded by Google SEO.