To make it an apples to apples comparison, look at submissions where the link submitted is the retail link to the IP. For example, look at all the book link submissions on AMZN...
I wonder if this is because the purpose of linking to a book is to share awareness of that book’s existence - nobody is about to go and read it then and there to comment on its contents. Whereas the purpose of an article is to discuss it now, in the comments - the consumption horizon and bulk of the content is different.