> it is simply a convenient identifier for underlying significant biological differences

It's actually not. Skin color does not correlate well with the genetic diversity among humans at all. It's just one particular trait that is very easy to identify by eye.

> There is absolutely zero reason one would rationally conclude that biological differences would somehow magically stop at the brain.

There is absolutely zero reason to rationally conclude that a random physical trait that happens to be easy to distinguish by eye correlates with brain function at all.

On the other hand, there are massive socioeconomic disparities that arise from the history of slavery, which easily explain both the disparities and the reasons why racists such as yourself want to boil things down to skin color.