Why do most African Americans have a lighter skin tone?

Than…?
People from Africa get more, and harsher exposure to sun than Americans.
Jamaica, the Bahamas, England, dark skinned people can be found worldwide who are lighter than persons living in Africa.

Because many of them are great grandchildren of slave women who were raped or had affairs with slave masters.

Plus, it depends who you are comparing them too. In the islands and parts of Africa and Latin America they might be in the sun more than in the US. Black people are darker when they spend a lot of time in the sun just like white people. Although, in Latin America they are very mixed also.

Keep in mind that all people tan. Black people get blacker! Mexicans get browner! And I suppose it may depend on how many of one’s ancestors spend lots of time outside.