I don't watch FFXIV content creators at all, but I know I've heard of massive viewer drop off when a lot of these streamers get through the MSQ. I think a lot of players like to see people's reactions. Of course world first races for something like an Ultimate are also some of the most popular, I think they are also the same in WoW. During EW launch, if that's how it was, I'd say it's because the people watching FF content are players themselves, and they were going through the story, so, yeah, they wanted to avoid spoilers, but they were also playing the game.
