Except that time happens and that single state policy is modified on a continual basis. These modifications are performed by coalitions of different parties with differing ideologies, leading to a constantly shifting "single ideology". Sometimes these parties and coalitions are formed as a direct response to consequences of the current policy.
Which is the point of a democracy. A democracy's central thesis is that any given policy is probably not-quite-right and that it always merits reconsideration in the near future.
> Real pluralism is about de jure authority and de facto power being distributed among many institutions in society, not concentrated entirely in a single one.
How is this not the case in a democracy? The assertion that country=government does not require that the number of institutions in the country/government be merely one. Asserting a distinction does not merely bump the number of institutions from one to two, which would be insufficiently pluralistic anyways. Pluralism comes from the liberal doctrine of individualism; it's a natural consequence of asserting the individual worthiness and dignity, which demands that the individual's concerns be considered in some fashion.
Making a false distinction between state and society is useful only for disempowering citizens who see themselves as part of one but not the other, negating their willingness to provide a voice to the cacophony of pluralism. Our disdain for politics is precisely what grants politicians so much power: because someone has to do their job and they're willing. If only the supply were higher so that the power granted by their office could be diluted by sheer interchangeability.
Why do we rely so heavily on the fourth estate to speak on our behalf? To be the voice of society holding accountable the state? Why are we ourselves not speaking? Yes, there are things stopping us, and we need to know what and why and how to remove those impediments. Instead of having our voices subsumed into mass outcries, we need to have our voices individually acknowledged. Pluralism isn't about having political speechwriters making dialogue with investigative reporters and mogul-owned pundits. It's about all of us talking.