They will not open this up for you until you have completed a certain amount of the application and/or they deem you competitive. The exams cost money and the military is trying to save every penny they can, thus they delay the exam until later on for many of the candidates.
~ It is also like playing chicken. They know many candidates apply to other branches, so if the AFA waits to request, but meanwhile USNA sent you, the Navy picks up that tab out of their budget and the AF gets the results for free BC it is like the CFA, they use the same exams.