TLDR: PhD studentships and post docs aren't best thought of as short-term jobs, though getting the dissertation done is the job of the PhD candidate and publishing articles is the job of the Post Doc. And the university and the supervisor aren't best thought of as employers. The university is a school and the supervisor is a teacher, and the PhD program is a specialized professional training program with a single aim - making new researchers.
I earned my PhD in the US, so our experiences might be a bit different, but I'm pretty sure the aim and structure of the academy is pretty similar across the globe. If you were my student, I'd say that this question demonstrates a serious misunderstanding of the role of PhD's and Post Docs in the academy and the relation in which they stand to the university and their advisors.
PhD programs are professional schools in which fledgling scholars undertake a course of study, which includes coursework, practicums, traineeships, direct supervision, formal and informal mentorship, participation in the peer-review process, academic enculturation, and individual research. PhD candidates are first and foremost students who are at the university in order to learn how to be academic researchers that understand the norms of the academy and their discipline, who have developed the skills needed to conduct research in their field, and who have the capacity to make novel contributions to the sum of human knowledge. At the end of this process, a new, matured researcher who no longer requires supervision and who can now, in turn, supervise new graduate students will have been produced. This new researcher serves to replace (and to build on the work of) existing researchers, as full-time academics at new institutions, as existing researchers resign, retire, or pass away. The PhD is a professional certification, like an M.D. or J.D., that signifies that the individual has met the basic criteria for performing the job of an academic.
Although PhD's typically receive payment for their participation in such programs, research assistantships and teaching assistantships are typically framed as traineeships for the PhD candidate to learn how to teach and conduct research - the basic jobs of the university-based researcher. The relationship in which the PhD stands to the university and the advisor is not best conceived of as an employee-employers relationship. Naturally, there are serious concerns about the treatment of graduate student labor in the academy - as previous commenters have noted.
If the university, the PI on a grant, or the director of a lab were seeking an employee, in the form of a lab assistant or project manager, it doesn't make sense to bring in a PhD candidate (although it happens), since the cost of educating a PhD is significant and the skills of a PhD are not needed for basic lab research. The B.A. or B.S. is a certification that the individual understands the basic research process or has a breadth of knowledge in a discipline that would allow them perform basic, repetitive research tasks. They are typically not researchers - they merely participate in the research process (a lit review is not "doing research"). The M.A. or M.S. certifies that the individual has specialized knowledge or skills that would allow the individual to perform highly specialized tasks in a particular research domain. Neither are research degrees and are not generally expected to conduct novel research on their own (though the UK has the M.S. by research which is a degree that might be thought of as a "PhD prep" program).
It makes even less sense to bring in a post doc for this purpose. Post doc positions are typically opportunities for the recent PhD to build a network of colleagues and references and to make progress on their original research, lending credibility to their candidacy as an academic, prior to scoring a full-time, tenure track academic position.
I will acknowledge that the norms are different in different disciplines. I'm a philosopher, and we are primarily a single-author discipline. So, my PhD involved no lab work under the direction of a supervisor. I didn't work on my supervisor's project. I set my agenda and picked my projects. PhDs in the sciences often work in grant-funded labs. Their research assistantships (i.e., research traineeships) often involves labor in the service of the PI and grant topic, in return for the use of a lab and supervision by the director of the lab. Work in the lab and on the grant generally contributes to the supervisor's research agenda, but the novel findings belong to the PhD candidate.