I reckon its more to do with server stability. If you spawn a full dungeon from the get go the way you do in WoW and other lower level dungeons - you will put a lot of strain on the server.

On a side note: Not everything needs to be a dungeon. I miss world content from FFXI and WoW. Specifically FFXI.
We already have a couple of content thats not dungeon based. Look at Odin and Behemoth. How much fun it is stacking on one map tile with 200+ other people that are spamming AOE on a small piece of land. Or how it would occasionally crash the server?

IMHO - they do not have a good handle nor a good solution to handle non-dungeon based content. Its not their fault per se since World events are hectic & messy. Unless something has changed or can be done i would refrain them from trying.

I understand where you are coming from - and i do root for them myself. But i would not trade them off for server stability/availability.