Because it isn't actually the most cost effective. Mass transit only works in areas of high population density. Otherwise you get empty buses or extremely long waits between service intervals. It's also, in general, slower because you have to wait for the bus and then be delayed as it picks up other passengers or takes an indirect route to your destination, which reduces the efficiency of ~everyone. There are also many others who need roads other than emergency vehicles: Delivery vehicles, tradesmen with their equipment, the proverbial soccer mom who has to transport the soccer team and all their gear to the game, etc.
And once you're already paying the cost to build and maintain the roads, you might as well use them for general purposes.
> Because it isn't actually the most cost effective.
That's simply not true. If people combined the money they spent on car payments, insurance, the opportunity cost from lower average lifespan due to car emissions and children being run over by pick-up trucks, and used it on taxes for busses, we could very easily have a high availability public transit system, even in suburban hell.
Though of course to achieve maximum efficiency the government must create tax incentives for dense housing and tax disincentives for suburbs.
Your points about busses don't make sense - a bus takes up 2 or at most 3 car lengths on the road. One nearly empty bus with three people in it is thus more efficient from every angle, including time because those people aren't traffic.
Your idea that point to point travel is faster with cars is false: come to Houston and I'll show you why (traffic). You can get around cities and even suburbs with public transit much faster for all rides. A great example is the train from the airport to an Airbnb in NYC. Only a fool would attempt that in a cab to save time.
In other countries, soccer clubs hire busses and vans for meets and the like, or individual children simply take public transit to events. This was true in America in the 70s as well according to my grandpa.
Building a road for emergency vehicles and commercial deliveries is one thing. Just needs one or two lanes. But for every person and their car: Houston. 8 lane nightmares.
> we could very easily have a high availability public transit system, even in suburban hell.
This is simply false. There is an unavoidable problem in suburbia: During parts of the day the number of vehicles that drive down certain roads is one vehicle with one occupant. Replacing a single-occupant car with a single-occupant bus is not more efficient. But running the bus only once every four to eight hours is unreasonable latency and objectively worse than the status quo where you can leave whenever you want.
> One nearly empty bus with three people in it is thus more efficient
You're assuming the bus will have more than one person on it. That's the unavoidable trade off. If there is one traveler every 90 minutes then getting a bus with three people on it would only be possible if there is only one bus every 4.5 hours.
> Your idea that point to point travel is faster with cars is false: come to Houston and I'll show you why (traffic).
Traffic is caused by bad design. Ironically it's the density separation that does this. You put all the density downtown but people live in the suburbs. Then there is no traffic in the suburbs but, because you need a car to leave the suburbs, unreasonable traffic downtown where everybody takes their cars, and on the main road that leads to downtown.
If people lived downtown then they wouldn't need to drive. But if every place was medium density instead of separation of high and low, you also wouldn't have a problem because some people could walk and the remaining traffic wouldn't all be concentrated in one place.
The problem is entirely caused by zoning rules and I'm not at all convinced we wouldn't be better off to utterly abolish all density restrictions whatsoever.
And once you're already paying the cost to build and maintain the roads, you might as well use them for general purposes.