Well, I did search. Definitely not a super-in-depth search, but still something. Both using Google and here, using the search feature of this forum.
The issue is that when searching for something like "4017 pinout <anything>", most of the results simply state what the chip pinout is.
I was also thinking that, perhaps, the pinout was chosen that way because that configuration allowed one to efficiently route the outputs out in several orderings. So I was hoping to find a picture showing how one could "sort" them in several ways thanks to their clever arrangement, whereas the real answer is much more mundane. Confirmation bias, perhaps?
This said, the first result containing the actual reason is not that far down the Google search results page:
http://letmegooglethat.com/?q=4017+pinout+reasonThat's why I said that I should have searched better.