Let me give you a datapoint and hopefully some intuition. I used to live in Calgary, which is an extremely car-centric city. Population is 1 million and city transit had about a 1000 buses plus two train lines, making it about 1000 people per bus. There are about a million cars in the city.
Let's say that the city got 10 times more buses i.e. 10,000 total. That's 100 people per bus, which should be adequate for everyone to use transit, even with the constraints of how bad the city design is (really wide roads, too many curves and deadends, spread out suburbia). Buses cost about half a million which is maybe 25x the cost of a car. So, the 10k buses would be equivalent to about 0.25 million cars in terms of capital cost. Which is a factor of 4 improvement on the number of cars in terms of capital cost.
Yes, busing people around in multiple occupancy vehicles does in fact reduce capital costs. That's why public transit networks in Eastern bloc countries were so extensive, often reaching every little town and village: communist economies couldn't afford to produce enough personal vehicles, so they ran buses instead.
But, reduced capital costs is evidently not what people in developed economies are actually necessarily after. Turns out, people are perfectly happy to spend extra resources to acquire exclusivity rights to a vehicle, because of flexibility it affords them. Having a private transport allows you to go where you want, when you want, and in almost all circumstances do it much faster than via a bus. The opportunity cost of time spent walking to bus station, waiting for a bus, and then taking longer to travel is for many people very significant, thus they pay for private transportation to recover some of that time.
Let's say that the city got 10 times more buses i.e. 10,000 total. That's 100 people per bus, which should be adequate for everyone to use transit, even with the constraints of how bad the city design is (really wide roads, too many curves and deadends, spread out suburbia). Buses cost about half a million which is maybe 25x the cost of a car. So, the 10k buses would be equivalent to about 0.25 million cars in terms of capital cost. Which is a factor of 4 improvement on the number of cars in terms of capital cost.