"* all of the factors you just listed also apply to railways"
massive Walmart style parking lots don’t factor if your urban planning is centered around public transit, and parking is definately one of the highest hidden costs of road infrastructure.
"* since railways are more expensive to construct and maintain than roadways, there are more cases in which a railway couldn’t pay for itself versus a roadway"
yes, when people stubbornly refuse to use rail infrastructure or when rail/transit infrastructure is prioritized less than roads/car based transportation then of course its going to be less economically viable. Economies of scale and induced demand are a huge factor here.
"* why would a company build a private road when the government will do it for them?"
good question, and yet we still have private roads and tollroads.