Humans write this way, too. That's why AI does. It has been fed a ton of human writing. It isn't much more than a predictive text machine. Garbage in, garbage out. But if a cliche comes from overuse, soon everything will be a cliche - because AI can produce content faster than humans writing in exactly the same ways human writing has been modeled for it. It's not "plagiarising" any more than you and I are, but it's also incapable of original thought, emotion, nuance... I'd like to say it's incapable of sarcasm, too, but it "learns" and mimics us quite well.

100% human writing can easily be flagged as AI (write a competent 5-paragraph essay and find out); 100% AI writing can also slip through the best "AI detection software" out there. We're just going to have to compete on the merits, and I'm not worried. Yet.