At least in computer science, postdocs have other benefits:

1) they can be well paid, like high five to very low six figures;

2) they can be an extra year or two to figure out your own research direction with some help but not much oversight;

3) they can be much easier to get than industry positions -- sometimes requiring little more than a solid publication record and advisor recommendation;

4) if you've been in a university environment for ~a decades and liked it, it might strike you as an easy path to keep doing that (this is probably the worst reason, though).

This is skewed by computer science postdocs at highly ranked schools, though. Yes, people taking these positions face opportunity costs, but the actual experience can be pretty nice.