When a writer uses “common knowledge” in an essay, why does it not require a citation nor is it considered plagiarism?

Common knowledge is a kind of knowledge which everybody, or almost everybody, knows. Because is not written in any place cannot be cited, nevertheless, it is accepted. It is not considered plagiarism because it belongs to a big part of the population, as a consequence, it doesn't have an author.

i think its 5049.

i hope this !