SV is suburbs by any standards. It's completely unservable for mass transit, so it's no surprise that there is little of that. Only SF, Oakland, Berkeley, and SJ have the urban density and mix of housing, commerce, and industry that makes mass transit work. Cupertino is never, ever going to have effective mass transit unless they bulldoze the place and start over.
You're absolutely right, but it's not just about density or suburbs - it is perfectly possible to construct suburbs that can be serviced by transit.
The problem entirely lies in the subdivision format of suburbs in much of the US - full of twists, turns, cul de sacs, and in general trying as hard as they can to inhibit movement. Getting to the nearest arterial road becomes an exercise in futility where you'd find yourself walking along un-sidewalked road for a whole mile before doubling back onto another street just to end up at a bus stop.
Low-density cities that are laid in a (saner, and less classist) grid are on the other hand generally well served by transit. This is no coincidence.
Mass transit and density need not be at odds with each other, though it's a sad fact that most suburbs will never be adaptable to sane transit.
