It's all a mix of fairness, amount of players and ragtack fictional history here.
For me, I initially didn't plan on including Korea into the Japanese Alliance at all, only after Panopticon announced we'd all start with 500 million people to keep it fair and reasonable fast I looked around Japan because I thought 500m living on the japanese islands is just ... too crowded. Even with the current ~130m they've got issues. Now with Korea 500m people in the nation is still surprising, but I didn't want to include any more territory for the start.
As for the annexation - things were
really bad. Along with the Invader threat of super killing machines almost destroying the entire human race they also had a new direct war with North Korea - that lead South Korea to accept japanese military aid as neither Japan nor South Korea have a nuclear arsenal, North Korea however conducted several tests and could arguable be in possession of ready-to-fire missiles.
The internal political scene of the Japanese Alliance is far from silent, I do want to play out the inner tensions and the favour the new nation has towards the japanese - After all most of the foreign affairs run entirely through Japan, South Korea remains under local 'provincial' government and North Korea is as a matter of fact occupied by combined japanese and south korean forces.
But the whole thing is just fiction I made up - I'm in no way an expert with neither Korea nor Japan. I had read a few sentences about Sakalin Island but didn't even consider claiming it as japanese. That can still become a fun matter with the Inuk.
With China and India - There's just noone who wanted to play them. I can understand that Panopticon doesn't want to play over ninethousand different factions by himself just because they would probably have survived - I'd count most of the 'gray' world area under disarray and local political rule and our 'played' factions as sort-of new super powers that dominate the scene. You could always take up either China or India to spice the game up