But the working class cannot simply lay hold of the ready-made state machinery, and wield it for its own purposes. --- Karl Marx /The Civil War in France

Marx goes on to say that the problem with the state is that it carries around the baggage of all the old forms of the state. The problem with just using the state for the ends of proletarian class power is that it retains the old class composition of the bourgeoisie, whose interests it was set up to protect. Therefore the "ready-made state machinery" must be smashed