America has plenty rules which aren't written down, too. You just lack a good word for them, having decided to use constitution to mean a particular bit of paper, which is periodically found to contain rights unknown to its authors.

A nice example is the slowly shifting norms around executive orders.

A bigger example: didn't both Liberia and Mexico basically xerox the US constitution? The bit of paper, I mean. For clearly the unwritten "ways we do things" in everyones heads were very different.