In the air, cars can drive on a 3d axis, have no way to see road signs, and are more likely to fall on top of other cars if they're lucky enough not to get into an accident with one already in the air.

Compare this to a tunnel, which has controlled routes, is out of the way of road-way cars, and if made properly will never collapse on itself. That's how.

Also, (and almost totally unrelated) Azimov predicted tunnels for autonomous vehicles in his Daneel Olivaw books.

Mind you, he also predicted "rolling roadways", like a big conveyor belt instead of trains.

The Roads Must Roll was Heinlein.

Asimov ran with it in Caves of Steel. But credit where credit is due.

Elevated roadways would be far easier to build than tunnels.

Not a structural engineering expert, but beyond a certain depth for tunnels (depending on soil type I believe) there isn't a structural risk to buildings above, especially at the tunnel diameter sizes suggested by Boring co.

With airspace you have several problems - noise, collisions, crashing on whatever is below.

Having said that, I think we'd all love a safe, electric VTOL vehicle. I don't think they would be allowed in densely populated areas though, same as drones above a certain weight.