The flight is not available in the planner because a specific provision of the RTW rules prohibits it.The rule, however, predates AA's introduction of flights to Cuba, so not sure if this is a case of an outdated rule not being changed or if it is still in force.
As you point out there are numerous flights from the US to various Cuban cities on AA and it is not all that difficult to comply with all requirements.
The best way to find out for sure would be to call the AA RTW desk.
Given proximity, it may be a more effective use of your 16 segments to simply book a separate ticket from Miami, which will also avoid the RTW restriction (if any).