Well,
it's not as simple as that.
Plagiarism
the process or practice of using another person's ideas or work and pretending that it is your own
Copying and pasting from an AI is not subject to copyright issues in many cases. In the case of the OP he is not simply copying and pasting something that the AI is making up, in order for the AI to produce that text he has had to give it a source of information from where the text has been generated. There is no plagiarism in the sense of the Cambridge dictionary and there is no copyright problem.
I don't think we can use that as an argument. How many decades or hundreds of years old is that Cambridge definition of plagiarism? It was defined at a time when there was no AI or chat bots, and it's no surprise that the definition only covers using another person's work. 'Something' couldn't plagiarize in those days. Now it can, and that something is the person who uses AI. As LoyceV said, the definition is outdated.
Moreover, according to search results, the definition for plagiarism dates back to the 1620s.