There are many students enrolled in university Ph.D., and most of them want to be professors. But if you want to be one, you need to start somewhere, and most new grads are stuck waiting for a tenure-track position at a university to open up.
If you've got a doctoral degree in hand, you've got a Ph.D. Unless a university employs you, this means you need a job—which means you need to get a postdoc or another academic job. A postdoc is a research assistant who will work in your lab for a few years, providing research and intellectual support. Unless you have an idea of getting a postdoc, a whole career in academics can seem out of reach.