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.