It's very simple really. No need to think too complicated. Suppose the PM is corrupt. Does the CPIB investigate him? Or is there a conflict of interest since the CPIB is under the PMO?
This is what happens when your totalitarian shithole regime does not have separation of power (
三权分立) and have one political party bao gar liao (
一党专政). You will always end up with the trashiest government.
And if you think that's a worthy sacrifice for 'prosperity' or 'stability' or whatever nonsense they constantly try to sell you (e.g. 'social compact')... you'll eventually find out the hard way that it is NOT a good deal.