So that we recognize that it is impossible for us to create the perfect society without God. Any government will be a failure if the people governed do not have faith; but moreover the Kingdom of God is not something which we create in this life. It is what we are waiting for, in heaven. That is where our glory lies, and that is where perfect communion is possible.
This doesn’t mean that we are not to strive to best prepare outselves and our societies for that glory–it’s that we cannot live as if we have already attained it. We cannot vest our spiritual leaders with vast amounts of temporal power because they are just as frail and fallen as we are; history shows that power corrupts.
This ‘Catholic theocracy’ talk is just as bad as the extremes of liberation theology, if you think about it. Though it goes about it in a different way, both are striving to eliminate suffering and injustice in our world to create a perfect land “flowing with milk and honey.” While that’s nice–we cannot have it yet! We dwell in a fallen world, marred by sin and death. Perfection is unattainable, suffering is unavoidable. The only way we can be saved from it, or save others from it, is by recognizing that at our core we ARE suffering, and that all that can save us is God’s grace. Christ is our King, and will be for eternity.
It is much more potentially fruitful to strive to put our faith into action in the political sphere–social charity–than it is to dream about a government run by the Pope.