Because you are extremely unlikely to get sick on the cruise. I know two people dying of cancer who were given permission to travel overseas by their Specialists (one to UK and one to NZ, which both provide medical to Australians in an emergency). Both had successful last trips full of happy memories.
Travel insurance, for a short period in Australia, is often overcharged for the risk. Travel insurance for my daughter who lived in London was much cheaper than mine in Australia, for the same trip!!