Because people are afraid of things they don't understand. AI is a very new and very powerful technology, so people are going to see what they want to see from it. Of course, it doesn't help that a lot of people see "a shit load of cash" from it, so companies want to shove it into anything and everything.
AI models are rapidly becoming more advanced, and some of the new models are showing sparks of metacognition. Calling that "plagiarism" is being willfully ignorant of its capabilities, and it's just not productive to the conversation.