Why are PhD programs so stressful?

It's a known fact that in order to get a PhD one has to go through years of stress, depression, humiliation, and financial hardships.

This is something I simply don't understand. I understand why surgeons may have to go through a lot of hardships to get their degree. I understand why navy seals may have it the rough way before they are sent on a sabotage mission. I even understand why lawyers may be under a lot of stress in law school. After all, peoples' lives often depend on doctors, soldiers, or even lawyers.

Why do you have to go through so much stress to get a PhD in, let's say English? To write a book that will sell 4 copies and qualify for a minimum wage job as a part-time lecturer??? There's just something wrong with the system and, unfortunately, the system is not likely to change within the next few decades.