I'm not sure if the developer necessarily takes a hit. Say I have $15 to spend on a video editing app. $10 for the initial app and $5 for the only premium feature I want results in me giving $15 to the developer (less Apple's cut etc). However if the developer adds 5 new features to the app and I have to choose between buying features that I mostly don't want and can't afford or not spending the money I'll most likey give $0 to the developer.

Similar situation with luxury features on cars: it makes business sense to sell them individually.