Because once you complete your class quests up to 30 you never see or hear from them again, excluding the Padjals.

The mog delivery quests will be perfect for this since it's optional and a perfect way to expand on NPCs and lore. Its a shame to relegate them to such a side role when some of them have such fun personalities and story to tell.

The Padjals are involved in unlocking some of the dungeons and trials and it was nice when E-Sumi briefly mentioned Sylphie's training before I killed Odin. And it will be nice if some of the other guild masters will be involve with atleast unlocking dungeons/trial, etc