It sounds like a fair number were not accounted for unless they were recaptured.
In a way there was not any real chance for them to get back to Japanese territory, or was there?
All were recaptured or died.
There was no realistic chance of getting out of Australia. For the Japanese, it was about their code of honour. Being prisoners was dishonourable - attempting to escape and dying in the attempt restored their honour under the Bushido code.