Looking back at World History, Isolationism has for the most part been a failed policy and mostly detrimental to the "isolated" Nation. If we treat each blockchain as a Nation then it would make sense to have as many inflows and outflows (Bridges) as possible to create robust economies.
I doubt the hardcore members of the HIVE Nation would "defect" and renounce their Citizenship on HIVE however it might be nice to go vacation somewhere exotic every now and then. I would encourage as much collaboration as possible. I do understand that many of the developers may not like other chains for various reasons including petty reasons like ego and pride.
The community on HIVE seems to be one of it's most valuable assets. I'm really not sure why every other Blockchain does not have a Community Page on HIVE. It's FREE advertising and a great place to get some secondary "Citizens".
Because they dont want to lose investors. They will choose a WEb2 forum instead of Hive dapps if it means that the investors, users dont use other blockchains.
Tribalism is at the core of crypto. The first projects that manage to find a way around it, and i proposed a simple idea in the comment above yours, will be the winners at least when it comes to market recognition imo.