That's the social housing waiting list. Of which more than half are single people (mostly) or two adults (fewer). No children. Been terminated from private rentals, including boarding houses, or asked to leave by flatmates? They might be perfectly good tenants, but a bit risky under the many new rules or landlords are making different choices with their assets.
Of course they are empty or soon will be. That is why they are advertised as vacancies. Then there are the thousands of ads for flatmates.
Yes that is their right. But there are also plenty that might be perfectly liveable, and cheap, but don't meet the new standards, so remain vacant. And those waiting out the 5 year bright line period before selling.
It is a complex sector with many aspects. Do not however discount the many changes brought in since 2017, in response to which assets owners make decisions in their best interest. Because they can.