The explanation is simple, twitter and facebook rely on ads to make profit. to maximize their profit they need to have people spend as much time as possible on their websites.

The most efficient way is to infuriate people. Hence the business model of these companies is to have people outraged and infuriated.

This is a similar strategy to using the media spread fear so people consume more. Simply said someone who's happy, live a fulfilling life free of worry does not feel the urge to buy and consume.