Why is novelty mandatory for a Ph.D. degree? The institute I am studying at has added the following requirement recently:


  A PhD scholar must publish two papers in his/her corresponding field
  in SCI journals.


This means that a student cannot get PhD without two SCI rated papers. To the best of my knowledge, novelty is a mandate for such journals; hence, novelty has became mandatory for a student to get PhD in my institute.

In this context, I have the following query: Why is novelty mandatory for a PhD degree?

Consider a scenario in which a student is ambitious and wants to do research on an open problem or famous unsolved problem in his/her field. The student cannot attempt to research such a topic since research may be locally saturated, and he/she cannot come up with a novel idea. Does the rule not restrict the research on such problems? I can see the alternatives like tutorial papers, survey papers, etc., but some of these require only highly experienced geeks.

Note: Assume that neither the PhD student nor the PhD supervisor is an extraordinary person in that particular field.
Novelty is a basic requirement in research because without novel results, the work is of at best limited usefulness. It's not useless - review papers are useful - but it's less interesting to the people at the frontier. In the case of a PhD, it is supposed to signify a student is now capable of doing research. Research (as opposed to literature review) necessarily involves new and novel results. Therefore novelty is a requirement.

If I understand your last paragraph right, you're wondering about open problems which might be too complex to solve in one go. But in that case, you still get new and novel results - they might be partial results, but they're still results. The same goes for research using new methods that turn out not to work: knowing where not to look is not as useful as knowing where to look, but by narrowing the parameter space, it's still useful.