40 cars are not equal to one bus, because not everyone are going from single point A to single point B. 40 cars go from points A1,A2..A40 to points B1,B2..B40, but those unfortunate bus riders have to use several buses and wait for each bus to reach their Bs and get back to their As. And if you have to serve people at least 80% good as cars you much more than one bus. And of couse nobody does that and that's why public transportation sucks.
Just an example: Moscow, Russia. Heavy public transportation, a lot of buses, trolleybuses and a huge subway system, often advertised as "transportation solution". Yet I had to buy a car 14 years ago, because commuting from my parent's home to work took about 15 minutes in car and an hour and a half on public transportation -- a bus to subway, subway, change line, subway, bus. A lot of time wasted in hot weather or freezing snow, waiting, waiting, waiting...