Plagiarizing content in the internet became a problem with the advent of adsense, which made it easy to monetize content. I have seen it all through the years.
I remember when the profitable thing to do was to scrub google searches and grab the top 10-40 results along with a paragraph of content from each while giving a link to the site. The culprits would slap adsense on the page and make money that way. Not many of those left, but I had a couple of hundred links to my website from places like that before.
Then I would find places where they would take content, change it a little and then publish it as if it was their own. This actually requires some work, and I find very little of that. Funny thing is I do not consider myself a particularly gifted writer, but having a finance related website makes me a target.
What is very prevalent is a lot of websites and forums that seem to have computer generated trash built around keywords and search terms. The good thing is that google and the other search engines seem to be very good at catching these and relegate them to the bottom of the pile.