Math can be greasy and messy. Definitions can be clumsy in a way that makes stating theorems cumbersome, the axioms may be unintuitive, proofs can be ugly, they can even contain bugs in published form. There can be annoying inconsistencies like optional constant factors in Fourier, or JPL quaternions.

Yes, prototypical school stuff like Pythagoras are "eternal" but a lot of math is designed, and can be ergonomic or not. Better notation can suggest solutions to unsolved problems. Clumsy axioms can hide elegant structure.