Facebook was originally about people you were acquainted with in real life. You had a pre-existing reason to engage with them. That engagement wasn't as lucrative as SV investors wanted, but it was there.
Twitter never had that premise, or lost it very early on. You screamed into the ether, and people either responded or they didn't. One way to increase the chance of receiving a response is to say outrageous things. Once people figured that out and how to put ads adjacent to the outrageous thing, there was at least some pressure on Facebook (later Meta) to do the same thing, because we're here to make money, not friends.
And really, there are elements of that in old media, too. Their business model was to have captivating programming on TV and radio that would keep you tuned in to see what was happening in the next part of the show after the ad break. Rush Limbaugh, Jerry Springer, every 24-hour news channel, etc. were all very good at this, coarsening of the discourse be damned.
Regardless of who owns it, if you introduce a motive to constantly and eternally increase the value of a media company, you will see a move towards slop content at some point if you have a long enough timeline. It's inevitable.