The Westwinster system has some distinct advantages. All leaders of the executive branch are directly answerable to the electorate. The Head of State is a figurehead whose absolute surety of office means she needn't be a political figure and so some parts of the machinery of government are more trustworthy. The fusion of legislature and executive means that the executive must be exposed to the view of Parliament at all times. The fusion also means that governments usually form quickly and so can govern with some confidence of expeditiousness (vs endless hearings), while bicameralism provides an inbuilt mechanism of resistance and review.

Meanwhile, Americans think the President is some kind of God-king. The powers a President has are granted by Congress, the money is voted by Congress. The President has enormous day to day influence and power, and could at an order destroy billions of lives. Yet the President has no guarantee of a workable Congress and Congress has no incentive to work with a President.

The only thing that can cause the Washington system to act collectively is some terrifying outside threat, and all they will ever agree on is the kind of madness that's led us to this pass.

So, actually? The Westminster system is pretty damn good, constitution or not. Americans think they're badasses for having a revolution. One. And a civil war. One.

Britain's constitutional system is more thoroughly underpinned by blood and terror and war and suffering than the USA's. By a long shot.