Because it's all been built.
It's called the road network. And it already goes door to door. That's the problem of course with rail and bus: you have to get to a station / bus stop so you often drive there!
But where are the actual public transport vehicles on this door to door road network, you ask? They're coming. They're called autonomous cars. They are going to be so sophisticated and so safe that many of us will abandon private car ownership completely. A fleet of autonomous cars - either government or privately owned - will constantly circulate the road network, available on demand to go wherever we want. Door to door. Traffic congestion will ease considerably because fewer of us will own cars. Look at all the private cars that currently spend the majority of their lives parked up somewhere. Not any more. If a car exists, it will be moving. There's nothing new in this. It's how air transport operates already. Planes basically never stop except for servicing and repairs.
Don't build Metronet yet, state government. It could become a massive white elephant. We should wait and watch. Things are changing fast...