I wonder if there are actually any statistics on this

Casey Perin, an associate professor of philosophy who directs the department’s graduate program, said 5+2 is especially appealing within the discipline because most philosophy dissertations are a series of articles, not book-length works.

source:

https://www.insidehighered.com/news/2015/06/23/uc-irvine-experiments-new-graduate-degree-and-postdoc-hybrid-program