Why don’t two Caucasians ever father a black person?

There is actually a condition opposite to albinism, called melanism.

Melanism is the exact opposite of albinism.

In organisms affected by this condition there is an excess of black pigmentation, unlike in albino animals, in which pigmentation is practically absent.

The best known case of melanism is that of panthers. “Panther” is just the name given to a melanistic jaguar or leopard. It is not a separate species, as some still believe.

It’s not clear if the same is true for a human, but theoretically it could be possible for two Caucasians to father a black person.