Why does HR even need to keep track of ethnicities? Is this a US thing?

Here in my EU country you're just employee/applicant #3215, that's it, nobody asks or enters your ethnicity anywhere to even be able to keep tabs on how many are of what ethnicity, since everyone is considered equal by default and judged exclusively on performance (in theory at least, in practice there are still biases, but tracking ethnicities won't fix that, since that's human nature).

What you're saying would even be against the law here since then it opens the door to bias and potential discrimination.