That is an EXCELLENT question.
For another chart of interest, lets take a look at teacher salaries over the same period, again in constant dollars. Did they triple? Did they double? Hell, did they actually increase much at all?
Teachers at my kids' schools had to spend their own money for supplies, field trip funds, and many other things, or beg them off us parents. And yet billions more go into the "education" system year after year.
What's wrong with this picture? Can any sane person see putting even more money into the system as the answer?