@hongminheeๆดช ๆฐๆ (Hong Minhee) I think it goes beyond that: if C2S was actually useful for building applications & didn't require heavy processing on the client, then it'd likely be more used.
But it doesn't define like "give me a chronological feed of all incoming posts" or "give me a chronological feed of all my notifications" or "give me all my follow requests that I haven't accepted or rejected"
Those are application concerns, which to implement with current C2S would mean fetching the entire inbox and/or outbox collection and filtering/reducing that dataset.
The consequence is that you inherently end up with either a backend for frontend (custom API / AppView) or a very heavy and potentially slow client application.