How 'pagan' the Arabs were is a contentious and debated issue. The audience of the Quran appears to be people well acquainted with the Abrahamic traditions
The answer to that question is not 'because it was plagiarised'.
Actually, much of it relates to non-canonical gospels and non scriptural mythology as well as the Bible. It is a discourse based on the late antique religious and political milieu, and in part, polemic, against certain religious trends - such as the polytheism of 'monotheists'.
You have a dictionary and can copy the definition of plagiarism, that is admirable. But, as I have explained, I disagree that this is an example of plagiarism. You are using a very simplistic and distorted concept of plagiarism.
All credible scholars seem to agree on this point. You and anti-Islam websites disagree with the scholarly consensus.
Without quoting a dictionary definition again, can you explain why all credible scholars avoid the term 'plagiarised'? Why don't they think it is accurate to use the term 'plagiarised'? Why is your understanding better than that of every major scholar in the field?