Wouldn't it also be much, much more expensive than building the line on the ground?
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...
I don't know the cost in US, but in my country Euro prices are: highway at 6 million/km, rail at 10 million, viaducts over 15 and tunnels over 20 million. The elevated rail would be at least 50% more expensive, probably more than double because in case of viaducts there is no restriction on what is below the construction and you can put pylons anywhere, if it is above and existing highway there are many limitations.
So it is doable, maybe a good idea if you have the money, but I heard California is burning money like there is no tomorrow and there are no funds for such a project (maybe not true, it's just what I heard).
Additionally, those highways typically enter cities, so stations could be built over them, often right in the city center. That does leave details about how those stations are accessed, but this seems minor since everything else has already been dealt with.
I wonder if an executive order could make it happen.