Because Pi is a defined constant, not a contingent quantity.
Indeed pi is a constant.
The Aristotle-Hawking side of the forum goes "yawn" - and dumps it into the anthropic principle with all the other mysterious constants, 'nuff said.
Meanwhile, the Plato-Penrose side of the forum wonder at the geometry. The mathematicians discovered circles - and their properties, they say, they sure didn't invent them. They were already there. But why? And why is it universal, the same everywhere?