My opinion: if you look at what the average American pays to have their own vehicle (fuel, maintenance, depreciation, insurance, financing), I think you can make the argument people are literally willing to pay hundreds of dollars a month instead of sitting next to people on public transit.
Would this be the case if public transit was better? Not sure which is obviously your point. I guess I'm trying to argue "the demand for not-public-transit is there, so maybe that's why public transit isn't improved upon"