A sensible person might say that such a government would have too much power. They might say that such a government is too large, its officeholders too far removed from the people to be just and fair. They might say that given the tendency of governments to accrue power, they might construe their power in such a way as to obliterate the powers, rights, and sovereignty of the nations under it. They might say that a republican form cannot properly function over so vast a territory, over a population with such varied and disparate interests. They might say that centralized power is mortally dangerous, and that once it grabs hold of power, it is extremely difficult to overcome. They might say that small, state-governments, representing small, parochial territories, and more likely to be just, and less likely to expand their power beyond all bounds. They might say that numerous smaller republics provide a safeguard against tyranny, whereas under one unified supreme system, there is nowhere to turn. They just might say these things.
Liberty is the objective. Not republicanism. Liberty is the end. Republicanism is the supposed means to secure that end. And as we have seen, republicanism, when improperly employed, can become an instrument of tyranny.
You still don’t know the purpose of government.
So you don’t think the US government should include a judiciary?