My answer derive from following definition of the PhD and its thesis.
The purpose of a PhD is to train a research student as an autonomous scientist
and a good researcher — i.e., as someone deserving the grade of Doctor.
The objective of a PhD thesis is hence to demonstrate to one’s peers that its author can be considered in this way by reporting the successful* completion of a high-quality piece of research.
Depending on one's field, the monograph-style thesis can be replaced by a set of published peer-reviewed paper. The objective remains the same.
The question is then "What is an autonomous scientist and a good researcher?"
Being a good researcher means first to do science well, second to do good
science, and third to do a lot of science.
Doing science well means following its ethical rules, being thorough, honestly and appropriately reporting one’s results, crediting one’s peers for their work and acknowledging one’s work limitations, using the adequate tools and methods to solve a problem, and doing research that is reproducible.
Doing good science means addressing a relevant scientific gap, an issue that is influential, a research problem that matters for science, the society, and the industry.
Doing a lot of science means to tackle a large-scale problem, to address subsequent research gaps, to contribute to multiple issues of a coherent sub-field.
One must, however, note that these criteria are conditional to one another. Doing a lot of science serves no purpose if one does not do good science. And more importantly, doing good science serves no purpose if one does not do science well.
To this, I would add that (it is my opinion, and one may disagree) the job of a researcher is to do science.
And doing science means reporting new knowledge that is as truthful as possible.
This is achievable using the scientific method that is commonly acknowledged as the best method for creating/unveiling (depending on your ontological positioning) reliable new knowledge.
So, to pragmatically answer your question:
- the purpose a a PhD is to train you as a scientist,
- producing new (and highly reliable) knowledge is quintessentially the role of a scientist.
Hence, you cannot be recognized by your peers as a scientist (i.e., being awarded the _Doctor_ title) if you don't prove that you can produce new reliable knowledge by yourself.
The confusion might come from the fact that — as it has been highlighted in comments — scientists do literature review, and literature review are useful and valuable. However, they are not the core of what is a scientist.
It it worth highlighting again the last quoted paragraph:
These criteria are conditional to one another. Doing a lot of science serves no purpose if one does not do good science. And more importantly, doing good science serves no purpose if one does not do science well.
Thus, I think that, when doing a PhD, one should mainly focus of the quality of one's contribution, rather than breadth or scale.
In other words: what matters is not that much what you have proven, but how you have proven it.
Of course I had the dream of a breakthrough thesis.
Yet I think that, during a PhD, it is more important to produce only a few robust, well designed experiment and well reported results on a minor "research gap"; rather than aiming to tackle a large scale issue without being able to produce robust results or reliable conclusions.
It's only once you'll have proven that you are a good researcher and autonomous scientist, that you'll be able to secure a job as a scientist/researcher.
And then you'll have time (and money, and support, and experience, …) to shoot for the stars.
Yet this is not the objective of a PhD. Everything in its own time.
Citations are excerpt of… the preface of my PhD thesis.
* Note that "successful" is to be understood as "We have completed each step of the scientific method". I.e., it does not consider whether results are positive or negative.