Is it really any worse tan a high-stress job? What is the baseline for the comparison? Is the corporate environment with all the office politics, performance reviews, project deadline pressure etc so much better?
IME, hard to say better or worse, but definitely different: less day-to-day stress (work on your own schedule, drive your own project, usually no one directly dependent on your day-to-day output) but much more existential-threat-type pressure (if this doesn't work, I won't graduate). Couple that with a culture that looks down on quitting, and the fact that one can't just "quit and find another job" while staying on the academic track (one can switch advisors but it's difficult and often costs years of progress), and PhD students are often under a tremendous amount of stress. Personally I'm happy I finished but I've seen it take a lot out of others.