Could be plain laziness or could be the local water authority didnt require one, which I'd find unlikely.
In the Syd. Water area it would have to be inspected and signed off aswell as the whole drainage system being re-drawn so they have it for future reference.
But unfortunately a lot of people dont bother, which makes it hard for future plumbers trying to locate a pipe that should be there, or finding stuff that shouldnt be there.