The issue is the inflation of requirements in some fields due to competition. A postdoc is generally understood to be further training in running your own lab a

The issue is the inflation of requirements in some fields due to competition. A postdoc is generally understood to be further training in running your own lab as a PI - you might manage and guide doctoral students, for example, or learn how to write and apply for grants. With that in mind, a lot of people are happy to commit to a few years of postdoctoral work. In some fields, however, the extreme shortage of tenure track positions has made very long postdoctoral stints a requirement, rather than as something to sweeten the deal. Like others have pointed out, in CS a PhD is enough to get you a tenure track academic position, while industry jobs abound. Meanwhile, in biology I commonly see 5+ years of postdoc experience being typical just to be in the consideration for an industry job that 10 years ago would have happily hired a fresh PhD. The general thinking is that a postdoc only makes sense if you want an academic position, but the glut of PhDs in the market has pushed up the requirements for all kinds of jobs.