I think it's probably because it's extremely hard to understand what *content* keeps users subscribing, and "hours spent watching this content" is maybe the lowest-hanging (or only?) "clean" metric you can use to determine what content to keep, how much to spend on content, etc. I watch a smattering of shows on netflix and so keep a subscription, but I don't watch very much. How the heck are they supposed to know whether a piece of content will keep me subscribing? And how much should they spend to produce/acquire that content? It's a very difficult supply-side question.