What do you mean by "the short window when the labor to do so was affordable"? Other cities in the world seem to be able to build underground railways just fine and they have similar labor costs as the US. See Paris or Sydney for cities that have created new underground railways recently.
But my comment was a bit tongue in cheek - it is mostly political dysfunction. Of course the US could find people willing to work for less than $400/hr or whatever, but there is an incentive disalignment.
Much of SF didn't even exist until the 1930s-50s. For example, most of Sunset and Richmond is tract housing built during that era - before then it was sand dunes and chicken farms.
People underestimate how new much of the Western US is. For example, Dallas only began expanding in 1891 after the railways were built, LA was a small town until the 1910s-30s era expansion, modern San Jose only formed in the 1960s-70s after absorbing dozens of farming towns like Alviso and Berryessa, Seattle was mostly sand dunes until they were leveled in the 1900s-30s).
Because of how new it was, most of the cities are planned primarily with cars in mind - especially after the 1930s era Dust Bowl Migration and the 1940s-60s era economic migration. Same thing in much of Canada and Australia as well, which saw a similar postwar expansion.
> before the NIMBYists stepped in
NIMBYism in SF only really began in the 1970s onwards.
While NIMBYism is now elitist, it initially started out as part of the civil rights movement ("urban redevelopment" was often a guise for razing historically Black, Hispanic, and Asian neighborhoods in that era - for example much of Japantown/Fillmore) as well as the early environmental movement (eg. Sierra Movement, Greenpeace), which was opposed to profit motive compared to modern YIMBY+Greentech model.
While that’s true of the outer communities (San Jose, etc) I took the OP’s message as referring to SF core/downtown which was already pretty developed by the 1950s. Unlike LA, SF was a major city far earlier.
Much of SF's core/downtown was rebuilt after the 1906 fire and earthquake, plus there was massive "urban redevelopment" that made the core much more car friendly.
People are forgetting about pasadena. That was the bigger socal city than la for a long time and maybe even bigger than sf (certainly is geographically).
