People spend more time on an app where they feel comfortable. If FB did nothing to clean up feeds, it would probably drive away both those eyeballs and the advertisers who don't necessarily want their brand to be displayed next to unfiltered user-generated content.
One might argue FB is trying to "clean up" a problem that it caused/promoted in the first place.
Over time they've changed the "feeds" to work on principles that are good for advertisers, "influencers", and viral shit in general... at the expense of regular human contact and normal trusted social circles.
I don't use the main FB app but when I have before it'd constantly show me stuff from pages/people/whatever that I didn't follow. I think it was mostly friends of friends, or stuff friends liked. Have they got rid of all of that now?
i just logged in to my account (i look about once a week) and everything i see that is not an ad is something i actively opted into or was shared by someone i chose to follow/friend.
Thing about Facebook is, it pushes stuff at you. Not stuff you'll like, stuff you'll _engage_ with. Last time I logged in it offered me a weird anti-trans Economist article. Life is too short to invite a robot to irritate you; that's why I don't use Facebook anymore.