Most of the ones I've known (from when I was in grad school and then from when I worked at a major biotech) do postdocs in order to build their research portfolio. If you want to a faculty research (not teaching) position in science, you need publications. These require research. Research requires time and money and in this day and age, the time typically spent in grad school is not enough to do a lot of top-quality research. And, grad school time is often spent teach undergrads, doing coursework, etc - whereas postdocs can usually afford to spend all their working hours on research.
So yes, postdocs aren't paid well, but most of that is because the position itself typically funds work that the postdoc needs and *wants* to do. It's a symbiotic relationship between PI and postdoc.
There are always, of course, the stars who are good enough to get research positions straight out of grad school. I've known a few.