[munificent]

The answer is obvious: Facebook loves misinformation.

Facebook's business model is fairly straightforward: they want you to spend as much time on Facebook as possible so you can consume as many ads as possible. Contentious, provocative content drives engagement. It's easier to provide an unending source of engaging content if you don't restrict that content to the set of things that are true. Truth has the unfortunate tendency to be boring in most cases.

If Facebook nuked all the lies, Facebook would be less interesting and ad revenue would go down.