Why do people say you shouldn't do a PhD unless you want a career in research? I know a PhD is supposed to be an apprenticeship for future researchers.  But what if you enjoy getting the PhD, but have no intention of doing research in the future?  It seems like being a professor is really stressful and there is a lot of competition, and same thing for other sorts of non-grad-school research jobs.  If you think this way, does this mean you shouldn't get a PhD, and why?
Research is not just the most important aspect of any PhD program worth the name; it's the entire point. Everything you do as part of the program (teaching duties aside) is to prepare you for a career as a researcher. So getting a PhD without any intention of putting this research training to use would be like spending years as a baking apprentice without any intention of becoming a baker: maybe not a complete waste if you really like baking (but not enough to make it a career?), but almost inarguably not the best use of your time. 

Lots of people get PhDs and don't become researchers, but if these people had known beforehand what industry they would end up in, odds are they could have gotten a better jump start on their career by getting a  Master's degree focused on that industry and entering the workforce after one or two years (rather than 5+ for the PhD). There are exceptions to this principle, but they are relatively uncommon, and usually involve industry jobs that somewhat resemble research anyway.

I can think of only two other justifications for getting a PhD: learning for learning's sake, and prestige. The former is a noble goal, but if all you want to do is learn without contributing to the body of knowledge in your field, this is contrary to the spirit of (good) PhD programs as stated above. Furthermore, you can learn all you want in your free time without entering a PhD program. Find out what the standard graduate-level textbooks are in your field, and work through them. If your knowledge gets up to the research level in a particular topic, you can start reading recent research articles. (Your local library will likely have access.) As for prestige, this may be subjective, but spending five years of your life for a piece of paper is unlikely to be satisfying in the long run. And, speaking as someone who has a PhD, you'll soon learn that most people will not really want to address you as "Doctor."

Having said all that, if you do like (or think you would like) research, but are put off solely by the expected competition and stress, I urge you to be more open-minded. Academia is certainly competitive and stressful, but so are most careers worth having.