The US can't realistically be considered an elected government until, optimistically, the 1960s. If you want to grant the US a couple centuries worth of mulligans, you can do the same with, at the very least, the UK, and arguably some other countries like Switzerland.

And you ignore the other key point, which is that the US had no real national competitors near its borders, while other countries whose stability you discount did. That's the key point to American stability, not any world historical genius in drafting the Constitution.