The only reason we would vote in a unified government is if we really want to get something done. Unifying all 3 branches of government can only result in extremes, and Americans traditionally have gravitated away from extremes.
We normally favor some gridlock because on any given issue the electorate is about evenly divided between the two extremes. What one party favors the other is somewhat compelled to oppose because there are no votes in taking a me-too position. That’s how the special interests are able to steer the political platforms of the two parties.