Depending on the school grad students are not necessarily cheaper to a professor than Postdocs due to the overhead/fees paid for students versus that paid for postdocs. Also the current academic job market in the US is bad. Taking a postdoc for 1,2,3 years has been very common in Computer Science for people seeking faculty positions in the last couple of years.
I've heard that it's possible (not easy) for statistics Ph.D.s to jump directly to faculty positions, but I think this is becoming less common. I think that economics Ph.D.s also often jump directly to faculty positions.
