To propose a less cynical alternative, high screen time means users are deriving a lot of utility from the service and are unlikely to end their subscriptions.
I've heard from Netflix employees that retention stabilizes once a certain number of hours per month are watched on an account.
I think that optimizing for time watched is genius for Netflix for exactly the reason stated above. Of course it helps drive retention, but the ultimate benefit is netflix-izing our culture so that a subscription is just necessary for our social life.
