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.
And in any case, there's no reason that public transit needs to be self-funded. We don't expect the same of most of our other public services.