Well you know, they do have to get the voice acting/scripting, cutscenes if any, programming the events in the raid, adding easter eggs and secrets, scaling up the enemies to the proper power level, squashing bugs, finishing geometry, creating loot box locations, tagging dead ghosts with the grimoire cards, fine tuning decals, polishing the whole thing together, etc. all of that takes time which needs to be paid to the employees. To liken this game to WoW which has a very similar history to Destiny, the devs need to be paid to make the new content, and since Destiny doesn't have a subscription fee, we need to actually buy the DLC. Just because the area might be there does not mean that it is finished.