Yoshi-P said he remade the game, rather than salvaging 1.0, so that we could have cross-world gameplay and a matching system to go with it. So you see, a homeworld-only Field Operation would go against that, and reduce the number of participants greatly.

Although it's just speculation, I believe doing everything in instances allows them to better control the number of participants ie. limiting to 72, which they can't do in the Open World. Although we currently experiencing scaling issues with FATEs still, in most contexts, the cross-world duty system prevents there being too many or too few like you get with hunts.

As for why there aren't dangerous enemies and complex systems such Logos/Lost Actions/Phantom Jobs in the open world, I believe it's because SE wants to keep the status quo for the people who just do MSQ then quit, because not everyone wants complex systems and challenging enemies. Instead, it's made something optional that people can unlock if they want that stuff.

Going back to Eureka, they were willing to allow the developers to make it really brutal in there provided none of it left the confines of the content (and it was, with enemies like Dragons 1-shotting people, and enemies being tougher than in OC on release).