Cars take up too much space. Relying on cars instead of mass transit means everything has to be more spread out, which makes walking and biking a lot worse.
They cannot.
> I don’t see how self driving cars would take up significantly more space than mass transit while on the road.
They would take up more space because they're cars. Cars can't get to similar efficiencies of space compared to buses unless you compared packed private cars with sparsely ridden buses. And then of course, there's trains.
Perhaps someday we'll have all self driving cars that can safely coordinate and pack themselves into smaller spaces on the road, but that's decades away at a minimum, since you'd first need to get rid of all manually driven cars.
However, a bus can carry a hundred people comfortably. Ten buses instead of a thousand cars results in much leas congestion.