The problems with Open World PvP in this game:
  • There is a lack of lore underpinning why we would do it. Why are Eorzeans killing eachother? That needs to be explained - all the city states are friends.
  • It would promote players to fight eachother, making them more toxic toward eachother.
  • Duels are restricted to The Wolves' Den and I can understand why. Duels can be really annoying in the world as people walk into a busy market area just to create a scene and annoy everyone.
  • I'm not sure if they can switch between PvE and PvP hotbars in the same zone but I expect they could find a way to do it if they really wanted to.
The way this game is designed is to make us work together to take down enemies, not fight eachother. Even the PvP lore that exists is us playfighting, doing military drills together. All the other PvP like Triple Triad, Lord of Verminion and Chocobo Racing are just good fun and nobody could seriously get toxic over those.

I think this game puts you into instances for most things on purpose, because any MMO that has a lot of open world content usually ends up very laggy with too many people. An instance allows them to limit that better while connecting people from other servers who want to do the same activity (such as PvP).