Not familiar with AB (or ADS-C come to that) but presumably since they are specific 'internal' positions relevant to your particular flight only there is no need for ATC to be aware of them? They really have no relevance I can see to anyone on the ground but I guess would be transmitted by ADS-C regardless if in the legs? I would assume that is the main reason.
Of course, putting them in the legs as waypoints also increases the number of legs and depending on SOPs, the amount of work involved for the secretary on the flight deck.