But there is also the ~80 staff manning stations at each end and along the route, and more staff in the control room, the cleaning staff, the people who manually do maintenance checks every single night, etc.
Your original point isn’t even true - in California there is constant maintenance of the road network and crews of community service workers doing their sentence cleaning trash on the highway shoulder.
In turn, that makes them cheaper.
Even worse for individual car transport. You need one staff (driver) per vehicle...