People in the thread are asking why people ride Waymo to SFO, which is well outside San Francisco proper. Thus, the whole peninsula's density is what's relevant.
The US does not have many metro areas with population densities above 3,000/km2. And those that do, like Washington D.C, NYC, Boston, Chicago, do have metro systems.
There are many urban areas in the US with population density of 3,000/km2 or higher that do not have any public transit at all.