I think the problem might be that plain words are still really spongy? A simple statement like "no vehicles allowed on public grass" isn't actually all that simple.

What happens when the words of the constitution apply to 95% of a given situation? 85%? 10%? Is it better to amend the constitution every time there's an edge case?

A lot of time you can just put a catchall textarea to cover all the rest of your input edge cases. For the law, that textarea is where all the action happens.