Based on my observations, particularly on search committees, I bet a lot of those 80% of doctoral students don’t want to be professors: scholars who are engaged in both research and teaching to some degree.
What many doctoral students want to be are researchers free to study subjects that interest them. For many, teaching, particularly undergraduate teaching, is seen as a necessary evil. I’ve seen it too many times. And I’m sorry, but if you don’t want to teach undergraduates, to me, you’re saying, “I don’t want to be a professor.”
It just so happens that professors are one of the few positions that allow someone to conduct research with a some freedom to follow their own interest. So a lot of doctoral students and post-docs hold their nose and pursue professor positions, because the jobs they truly want – independent researchers – are in even shorter supply that tenure-track professor positions.
Additional: See comment by Dave K. re: correction.