Buses take as much space as 3 cars. Even if you can manage to optimize everything perfectly to pack 4 people in these cars at all time, you only move 12 people using the same amount of road of a bus transporting 60 to 90 people, much more in articulated vehicles. There is just not enough room in dense cities.
You have to consider the overall efficiency of a bus network, not one particular trip. In urban centers where buses are run on frequency, not timetable, a transit line is attractive because it runs often. People take the bus in rush hours in part because they know they can depend on it all day. These seemingly low-efficiency trips where the bus is half empty are actually very important to the overall efficiency of the network, because they induce demand.
The case for transit is a space argument, not a fuel argument. Mass transit is simply the most space efficient way of moving people around.
Actually diesel busses are only "efficient" when they are full. They're barely as efficient as single occupant cars just on fuel use when you factor in the bus running underutilized much of the day. That's not counting the cost of the driver, and assuming bus capex is efficient.
So even a commute-sharing system like that being tested in Waze would be more efficient than busses.