Actually I think a lot of areas / organizations could really be fleshed out by APs more than hardbacks. Want to really see what the Hellknights or Stewards do? Here's three books where you can play as them and get an inside look. Want a better look at Vesk Prime? Here's a book set there. Etc.
For some things, I agree. In fact I think one of the Aeon Throne, or maybe its Signal of Screams, but one of the backmatters in one of those books is about the Stewards, and of course there is backmatter for the Azlanti Empire in the Aeon Throne AP.
But the Veskarium's core system is 8 planets plus whatever external planets/systems they may have conquered. That is too much info for the backmatter for 6 volumes if there was a "Veskarium" AP.