They would also lose a large number of users. It wouldn't look good when the stockholders get a presentation saying that the user base shrunk, because the moderators kicked out a significant part of the users. Consistent moderation means kicking out anyone exhibiting that behaviour you just punish one person for.

And that's why a lot of social networks originally ended up in this mess. That's why there is so much controversy and so many personal attacks and flame wars.

Because at the end of the day, numbers mattered to them more than a decent community. It's kind of like why the media is so polarised and broken now. Because getting your readers to flame you and each other to a crisp gets page views and ad clicks. It's easy to sell junk that preconfirms your followers personal biases.