What? Autonomous cars can park wherever...

Only by taking up more road space going to a parking lot. And more energy in those travels - that energy is at best 50% fossil fuels.

But they take up much more space than busses/trains per passenger.

So what? It's a genuinely irrelevant detail...

We don't need to optimise for space, we need to optimise for cost of transport, and door to door duration.