The answer mostly is about when they built them. My guess is this was from the 50's or 60's on a lot of this stuff, when the amount of suburbs was still low which limited the costs of infrastructure spending on maintenance. Now we can't afford that because we have zillions of miles of roads that have to be paved and kept up for low tax providing single family homes. The density of these houses is too low to justify tunnels/bridges all the places we need them.
Then add in modern cost disease on top of it and it starts making sense.
Then add in modern cost disease on top of it and it starts making sense.