But maps, fates and hunts happen on a specific world, whereas duties do not.

There is a massive difference between a cross-world party and a normal party.

A normal party can share loot and enmity, whereas a crossworld party is as if each person in it is solo. Sharing loot and enmity is important when doing maps or fighting hunts and fates where the enmity affects the rewards.

So if they made hunt parties cross-world, most people would only get 1-24 Sacks of Nuts from hunts instead of 40 or 100. You wouldn't be able to roll on loot for maps.

It would be convenient if these parties could be advertised across worlds, but still remain same-world parties and still require them to switch worlds before joining.