in many fields you publish 3 papers and staple them together for a thesis

The bureaucracy involved needs a way to check that the phd candidate is doing decent work (preferably something more objective than the promoter's say-so), and the scientific peer review process can be used for this purpose. Thus, phd candidates are often asked to produce some amount of papers and publish them (sometimes in journals with a specified minimal impact factor). Knowing how much work goes into the production of a paper, and how long the review process can take ( > 6 months is no exception), it would be unreasonable to also expect a fully original thesis. Ideally, but not always, the thesis expands somewhat on the previously-published papers.