I mean, that literally did almost happen once, right here in the US of A. And sure, points to you, no country has yet attempted it since then, but also, China has locked onto a much more productive and efficient model; controlling the flow of information itself, rather than the client devices at hand.
And yeah, that's circumvent-able by users of sufficient tech literacy and bravery, but also, that fact has not presented an issue yet for China's Communist Party in their efforts to keep a stranglehold on the culture of their country, apart from perhaps Hong Kong.