[mayd]

Pi is no more of a human construct than any of the integers. Pi is inherent in Nature as are the integers, but more mysterious. If you accept the existence of the integers then infinity exists and therefore pi exists too; it does not need to be constructed.

[adastra22]

There are no circular objects in fundamental physics.

Pi shows up in many physics equations, but that’s entirely due to our choice of units.

[eigenket]

There may be no circular objects, but thats mostly because the universe isn't two-dimensional. There certainly appear to be spherical objects (to varying degrees of approximation), and pi pops out quite naturally when you ask questions about the geometry of such fundamental objects as a black-hole.