As I said, you're confusing power and influence
George III (before he went mad) had no political power.
So you admit that the USA's political system is not used by ANY Westernized democracy
Australia, NZ ans Canada all use a parliamentary system and they're younger than the USA
Japan also has a parliamentary system with a prime minister and an emperor as head of state
The presidential system of government doesn't work well even for the USA, it's just the system the founders stuck the USA with
Have you not heard the term "parliamentary democracy" ?
"A parliamentary system or parliamentary democracy is a system of democratic governance of a state (or subordinate entity) where the executive derives its democratic legitimacy from its ability to command the confidence of the legislature, typically a parliament, and is also held accountable to that parliament."
Parliamentary system - Wikipedia
You might be thinking of the UK which has no written constitution.
Other parliamentary democracies eg: Ireland, Germany and Japan do. So their parliaments can't just strike down a part of the national constitution
You've never studied politics beyond high school social studies have you ?