The key phrase being "in my experience" ;) That's not a the result of an in depth study, merely personal experience. I just don't think I've met many (or any, come to think of it) people who could state unequivocally at the start of their PhD that their goal was to attain professorship - most people assess and adjust their goals as they go along, and maybe a purely academic career works out for them.
Plus, people doing PhDs are (hopefully) not stupid - they have a pretty good idea of the proportion of professors to PhD students when they sign up - I should hope they can do the math slightly better then your average gang member.
In my personal experience (at a top humanities PhD program in the US), everyone started out with the explicit goal of becoming a professor. One or two did, and everybody else recalibrated their goals along the way.
We all knew the statistics, but every single one of us assumed those numbers were about other people.
Plus, people doing PhDs are (hopefully) not stupid - they have a pretty good idea of the proportion of professors to PhD students when they sign up - I should hope they can do the math slightly better then your average gang member.