They were added later since it was plausible(balance wise because they're jse and lore wise cause Balrahn gets around)
They *could* always ret-con some RUNs and GEOs coming into the Mainlands. Such as groups of them sneaking over into the mainlands right around the time you kill Shadow Lord and gain entry into Dynamis. Within than time, they got dragged into the parallel realms and were overwhelmed by Beastman Hordes, their despair manifesting into either Fomors or spirits of the fallen beastmen.
on that note, they could do the same with the previous expansion jobs, too.