There are several reasons people may be part of the Shouting Class:
Reason 1: Idealism. Some people feel an emotional need to feel like they're improving the world. Since the world is never perfect, and fighting for a better world is intrinsic to these people's motivation (and probably their identity), they will always continue to speak up. Notice that there are many, many idealists who are not part of the Shouting Class - some express their idealism by building homes for the poor, or volunteering at an animal shelter, or working as a civil rights lawyer, or being a politician. Shouting Class idealists are only those idealists who see shouting as a key way to bring about positive change.
Reason 2: Personal Unhappiness.
Research shows that negative moods make people much more likely to engage in online trolling. We also have good reason to believe that some people are just generally unhappy people - though life events make them relatively happier or sadder, their baseline is a negative emotional state that changes only very slowly. In other words, some people join the Shouting Class because they are giving vent to the negative emotion that they are constantly experiencing for reasons mostly unrelated to the problems they're complaining about.
Reason 3: Sadism.
Research shows that many trolls are sadists, who delight in making other people feel uncomfortable. Since recipients of complaints and negative feedback often feel uncomfortable, joining the Shouting Class can be a way of indulging
sadism.
Reason 4: Argumentativeness. Arguing is an intellectual activity, and many people enjoy intellectual activity. Some people enjoy argument
specifically, while others just use it as a break from other kinds of intellectual activities (writing code, etc.). A subset of these people are "mansplainers" who just want to show off how clever they are or listen to themselves talk.
This is probably not an exhaustive list; I'm sure you can think of others. But it's pretty clear, from research and from personal experience, that there are a few people in society who fall under one or more of these categories.