Facebook generates revenue by selling ad space on its website.
Ad space that is surrounded by attention grabbing content will generate the most revenue, thereby maximizing shareholder value.
At the lowest common denominator, negative emotions such as outrage and fear draw attention.
Facebook has no incentive to remove or alter this content in any way, other than to avoid illegal content that could produce lawsuits.
Fake news is not illegal.
If you don't want to consume the same questionable news as the lowest common denominator, pay for a subscription to a news service. Otherwise don't use Facebook for anything more than a photo sharing/messaging platform.
Dis-aggregate the pieces of the platform. If you can't do that, then avoid the whole service. There are plenty of alternatives for each piece.