For example, a lot of people get knowledge of things from television, friends, parents, family, local newspapers, blogs, teachers, etc., all of which are unacceptable or difficult to reference.
Maybe you only learn things from proper sources, but if so, you would be the exception.
I know plenty of things that I haven't got from reliable sources. I don't use that knowledge to write Wikipedia articles, though. Writing an article based on personal knowledge violates our policy of only using reliable sources.
We didn't used to worry about sources, which is why we have a large number of unsourced articles. For those, we have no choice but to find sources after the fact and add them in. Newly written articles (and information added to existing articles) should come directly from a reliable source and that source should be cited at the time the information is added. We need to stop being so tolerant of people adding information without citing sources.