Marketing PhD candidates write papers. However, some design PhD candidates publish books. I have received book-format theses from two people who obtained their design PhDs from European universities. I also know of some other design PhD candidate in US  who are presently writing books for their PhD degrees. Why do marketers and designers require different formats of work to obtain PhD degrees?

I believe that design PhDs are asked to have a broad understanding about an area, whereas marketing PhDs are asked to generate specific piece of information from an area. An author of a book (PhD in Design) raises a broad question (e.g., value of design thinking), reviews others’ answers comprehensively, and then makes his/her own argument. Differently, an author of a research paper (PhD in Marketing) raises a specific question (e.g., value of design thinking is greater when economy is good than when economy is bad), reviews others’ answers briefly, and then test the question by performing statistical analysis. In sum, design PhDs make a holistic approach whereas marketing PhDs make an analytic approach.