To have an existing landline, you would have been credit checked by Telecom. Therefore reducing risk.
The other side of it was that credit checking was expensive and difficult to automate back then.
We now do credit check, and it is automated for our products. Just need to sort out lighting up brand new connections.
In terms of why, we simply haven't got round to it yet. It is on the list, I agree it makes no sense in this day and age.