Why: grad school is a great time to be independent and work on a lot of things that you want. But, after your 6th-7th year in the PhD program, it will quickly flip from being the best time in your life to being one of the worst times of your life, if you can't manage to graduate.
You will question yourself. You will envy your friends who've graduated from the program and are now working as a professor, or for Google or for MSR. You'll look around at all the new grad students and realize you don't know their names.
So the quote is right on. Enjoy your time, but get out as fast as you can. :-)