Which is exactly how all human artist actually work too, do you really thing that human artist suddenly imagine something totally original out of thin air not at all based on things they seen or experienced before ? Do you think that every time a human write a story, a song or create some image this person has asked for copyright all the previous artist / author that might have inspired them either directly or subconsciously.

Of course not, peoples see, hear, experience things, be it on the Internet, in books, painting, on TV and in real life and then reuse it that to create their "original" art.

For example look at how Star Wars was created, it wasn't a fully "original" idea from George Lucas pulled out of thin air, it was based on the real world events at the time and his liking of Templar knights stories, as in, it was based on somebody else stories, that he adapted / altered to create his own "original" stories.

That's not different of how AI are working, they are just pattern recognition algorithms, they are not "copying" other peoples works, they are identifying patterns that they then recombines into something news. Again very close, less efficient but on a larger scale, to what humans have been doing since the beginning of time.

Calling that plagiarism is like calling all recent movies,TV series and book plagiarism because they 99% of their stories are based on variation of existing tropes or elements found in other stories or real world events.