Having multiple people on the lease is not a strange practice in Australia; it's normal and not as you described. When signing the lease you're not signing for a room, you're signing responsibility for the rent and condition of the whole house/unit (excluding responsibilities remaining with the landlord, e.g. maintenance). Each of your housemates are too.

Rents are expensive. It makes no financial sense to live by yourself.

Even in my home town - which has no substantial university - it's generally assumed that if you rent and do not have dependents, you almost certainly have housemates.

Interesting. I would guess that the general economics have forced the landlords to accept and perhaps even promote this scheme, as it's the only was they can find a market for their services.

I wonder if this acceptance will keep once the economics changes?

I think you have it the wrong way around - it's not a demand-driven arrangement, but a supply-driven one; landlords prefer to have more than one person responsible for the lease, provided the tenants have sound integrity.

It's like standing on one leg; if that leg crumbles, you will fall and land hard. But if you have two legs, one leg can crumble and the other gives you enough time for you to land gently. If you... magically had a third leg (and weren't standing with them in a triangle?), you might not even notice that one leg stopped working.

If that's the case, why hasn't this naturally occured everywhere? Why only in Australia and for students in university towns?

I don't understand why you think this is unusual. At least in the areas I've lived in (LA, Silicon Valley, San Francisco, Seattle, …) it's been very common.

This is the norm in the UK too for shared housing. You have multiple named tenants on the lease and each is responsible for the full rent if the others can't pay.

Rents in Australia have followed the exhorbitant house pricing, and these days if you want to rent, you either share a house, be a well-salaried person, live in a dog box, or live in the boondocks. There's no such thing as a single person on a median or lower wage renting a house in a nice area anymore.