The way Twitter works seems to bring out the worst in people in a way that say Facebook and Instagram do not (the mass pile-on phenomenon is enabled by Twitter’s functionality. It’s much harder to create a mob on other platforms).
To this - and also to the question posed back to my first response to this thread: nothing's stopping them except, as correctly pointed out, the fact that it's not in line with the business model. And because there are some legal issues involved. Basically - and, yes, gross simplification - if you moderate too much you might end up accidentally making yourself a curator of content rather than a simple publishing tool for other people's content - and that puts you in a worse legal position. Which is insane but obviously also a major incentive.
I have to strongly disagree with the notion that Facebook isn't part of this particular problem, though. As a matter of fact, some of their content is so utterly vile that they were successfully sued after some of their moderators literally developed PTSD in the job, and major tech companies including Cognizant and Accenture have come under massive criticism for the working conditions of moderators employed by them. Facebook is less fast paced than twitter, so you won't get the exact same insta-pile on. You're also looking at a somewhat different demographic. But the content itself is often just as horrifying.