Racism in the USA

A bit of a controversial topic and I won't leave this post up for a long time because I don't want to cause troubles. I am curious as to why the USA still uses race classifications from the 1700's. The rest of the world identifies by their nationality. In the USA, no one is identified by their nationality, everyone gets placed in a category by skin colour. I know a lot of people in different countries also get confused by this because two people can look similar but are from completely regions with different languages, religion, and cultures.