That's because AA.com defaults to BA for service into Africa; the fees are BA's extortionate "carrier imposed fees" on award tickets. (Used to be a "fuel surcharge" but BA's dropped that name after being sued in US federal court.)

You can't book it on AA.com, but can by phone - AA now allows connecting to Africa through Doha using Qatar Airways; QR serves ADD, NBO, DAR, JNB and CPT.

In your case I'd look at using AA miles (NOT on BA) to get to and from Africa via DOH, then Star Alliance miles - Ethopian and South African - for travel within Africa. If you need to use Kenya Airlines, you can transfer MR or other points to some Skyteam member for those segments.

It's a long trip - I would absolutely try to ride the transatlantic/long haul segments in business class.