Quote Originally Posted by Felis View Post
Lorewise you enter the underwater entrance of the dungeon alone.
And you're alone when Alphinaud and Arenvald find you later on. Your party members were never there.


Quote Originally Posted by kikix12 View Post
The only times the game assumes you are in a party are [..].the trials usually.
The trials are actually the time where it makes the least amount of sense to have a party in, regardless of Lyse and Alisaie telling you to put a group together for Susano. Logically it makes no sense you'll leave them in the middle of enemy territory where they're heavily outnumbered and get a group together, then make it all the way back to the vault to fight Susano, who was said to be a Primal so you'd have to find Echo-protected members on top of it all. And where are your allies after the fight is over? Nowhere to be seen. Garuda - you're there with Cid and Alphinaud only. Ifrit - you were captured with the group of Flames, no party. Thordan - no party members otherwise the scene after makes no sense. Ravana - only you and the Scions there with Shiva getting a beating. The list goes on, and it makes sense you're not bringing a party with you because there aren't supposed to be that many people with the Echo in the game to begin with.
There are time when it makes sense to have a party, like Sirensong - your party members could've been on the boat with you, or Doma Castle, even though the scenes after that show none of your party members so that kind of debunks that. Bardam's Mettle - you're supposed to tame the Yol on your own, and it's one Yol per person, no reason to go in there with a party of four, and then you're alone again after you leave the dungeon. And the list goes on.
tl;dr this is an mmorpg so most instances are party content, but the story ignores that almost completely.