Real humans write like that though. And LLMs are trained on text not speech. Maybe they should get trained on movie subtitles, but then movie characters also don't speak like real humans.
"LinkedIn Standard English" is just the overly-enthusiastic marketing speak that all the wannabe CEOs/VCs used to spout. LLMs had to learn it somewhere
No, they do it because they're mode-collapsed, use similar training algorithms (or even distillation on each other's outputs) and have a feedback loop based on scraping the web polluted with the outputs of previous gen models. This makes annoying patterns come and go in waves. It's pretty likely that in the next generation of models the "it's not just X, it's Y" pattern will disappear entirely, but another will annoy everyone.
This is purely an artifact of training and has nothing to do with real human writing, which has much better variety.