I think the premise of this question is both incorrect and probably racist in the technical sense of the term. Not all African Americans have "lighter" skin and not all Africans have darker skin. This is a picture of an extended family in South Africa, which has various shades of skin tone. This is a picture of an African-American family displaying similar variations in skin color. In both cases, some are lighter, while others are darker.

With the exception of the rape of African-American slaves by white men (which is, by definition, forcible breeding) and the general push towards high birth rates among slaves, I'm not familiar with any concerted efforts at forcibly breeding slaves.

What caused African Americans to look so unlike Africans?

Do they?

They have a much lighter skin tone

Is that always the case?