Why isn't New Guinea included in Oceania?

All of Papua New Guinea is included in Oceania. Papua New Guinea occupies the eastern half of the island of New Guinea.
The western half of the island of New Guinea belongs to Indonesia, which is regarded as part of Asia.
All of Papua New Guinea is included in Oceania. Papua New Guinea occupies the eastern half of the island of New Guinea.
The western half of the island of New Guinea belongs to Indonesia, which is regarded as part of Asia.