Job Recruitment Website - Ranking of immigration countries - Papua New Guinea and Indonesia are on the same island, but why didn't they merge into Indonesia?

Papua New Guinea and Indonesia are on the same island, but why didn't they merge into Indonesia?

Indonesia has n islands, with more than 10000. Papua New Guinea is adjacent to Irian Jaya province in Indonesia. Where on the same island as Indonesia? There is no merger because it is a British colony and Indonesia is a Dutch colony, so there is no merger after independence.