This obsession probably comes from America's blood quantum laws, but also we are really a melting pot in many ways.

We have no real history or traditions so we look to our genetic makeup to help us build an identity

https://en.wikipedia.org/wiki/Blood_quantum_laws

My Australian Aboriginal friends were confused by the quantum thing, like to them they were either Aboriginal or not, they didn't care much about percentages. And from the other comments and meme, I'm guessing most other people don't either