Because the scale makes this nearly impossible. Or rather, extremely expensive to the point where only the biggest of companies can do so, and at the cost of real-time information.
How often are people bootstrapping a social media site? That's not something you rollout with a tight budget. Most websites do not allow user-generated content. This won't have nearly as big of an effect on the sector as you think.
Whether they are paying people or writing automated systems to remove content they disagree with, these companies argued for this legal protection on the grounds of protecting free-speech, and now that they want to restrict it they don't deserve those same protections.
