Serious question - keep aside flying cars and underground tunnels. We know materials science has advanced significantly over the past few decades. Why can't we

Serious question - keep aside flying cars and underground tunnels. We know materials science has advanced significantly over the past few decades. Why can't we figure out a way to suspend/elevate (using strong cables or beams) large tubes 40-50 feet above us -- at least along major arteries that are free and clear of buildings and other overhead structures. Dedicated vehicles or conveyer belts or some such mode of transportation would ply in these tubes transporting people, with elevators to get people up and back down. As compared to flying cars and tunnels, why wouldn't this be more cost-economical and speedier?