You hardly have postdoctoral positions in math, physics and social sciences. Lots of people still manage to get an assistant professor position after PhD. Postdocs emerged only recently and they are a buffer for those who cannot get a TT job after PhD. In biology it is absolutely normaly, in fact necessary, to go through ~5 yrs of postdoc before dreaming of applying. The estimates I know about math is that 1/5 of graduated get a job in academia. In biology is about order of magnitude more difficult (some less backed up estimates even claim is 1/300).
"You hardly have postdoctoral positions in math, physics and social sciences."
I don't know anything about the social sciences. In physics, I know that postdocs are absolutely necessary and expected, to the same degree that they are in the life sciences.
Mathematics is a little different. There are a very small minority of prodigy-types that go right from graduate school to the tenure track. However, post-doc's are still the norm, although they don't go by the name 'postdoc.' Usually they are pronounced 'visiting assistant professor' or 'instructor.'
Here's an example of a 'postdoc' in mathematics. All of the big universities have them:
