Until they're useful for making a catchy address, like me.ga (which wasn't hosted in Gabon)

Many region level tlds require a legal presence. .au requires you to provide an Australian Business Number to register.

It’s mostly the tiny islands that decide to cash in on fancy tlds who don’t care so much about providing them as a service to the small population.

It's pretty much never checked. You can search for a random name on https://abr.business.gov.au/ and use it.

It’s in the ToS though. You can register and at any moment you risk having your domain taken. Especially if you are a large business