Because there isn't a chapter limiting code for the buildings offered in the MA. Nor do I want one - imagine the devs deciding no one below chapter 9 should have a Moonstone Library? Or only those in chapters 1-4 should be able to get any of the codices, unicorns, or rainbow flower cages?
No, no. Let them put the Orc Nest in front of me before Orcs, and I'll happily wait until the next rotation to see what new building is offered. I had one. It made nice T3+1. I don't need it now, so I sold it. Put in a prettier building (which also produces T3+1, incidentally). I'll make another when I get into Orcs, and handle those the way I handle the GoOS - nuke it when I build a new one for the new chapter.