Moreover, the public debate over student loan forgiveness typically fails to mention that graduate students owe 50% of all student loan debt, despite being only 25% of all student loan debtors.
Getting a PhD often takes many years, and the PhD completion rate for incoming graduate students is very low. Even after you get a PhD, it can be difficult to get a tenure track job. For those who get on the tenure track, you'll probably be fine financially, but for everyone else... going graduate school can ruin your life. You may waste the best years of your life while accumulating a debt you can never pay off, without even an official graduation certificate to show for it.
College, including graduate school, is so much more expensive now than it was decades ago. And people forget that the post-WWII generation could use the GI Bill to pay for both undergraduate and graduate school.