I'm not American, but I wouldn't be surprised if there isn't much appetite for building transport infrastructure above existing transport after the Oakland viaduct collapse in the 1989 SF earthquake. I'm old enough to remember it, and watching from abroad, it was pretty horrific.

The cost of building something like that is likely to be prohibitive because you'll need to dig support pylons by the road and each and every pylon would need a ground survey for existing utilities, and so on. It adds up really quickly, even with a concrete factory on site manufacturing identical parts (which they do for tunnel building for example).

https://en.wikipedia.org/wiki/1989_Loma_Prieta_earthquake#Oa...