This would waste something like 250 million person-hours (or about 350 lifetimes) of riders' time every year, more than the total number of annual deaths. It would also add to operating costs and decrease capacity on overcrowded lines.
That seems like a pretty low bar for "almost impossible", as I would expect most HN readers are both 1) highly educated and 2) single, and so the two avenues you mention are amongst the most accessible to them.
There are far more difficult places to gain citizenship (e.g. Liechtenstein, China).
- There are certainly high rises in downtown SF that comfortably fit 8000+ people.
- Tech company employees live in the south bay mostly because that's where the tech companies are, not the other way around.
- Reaching SF from the east bay is much easier than reaching Mountain View/Palo Alto from the east bay; in general SF seems like a better compromise for commute access from the whole region.
- Not requiring parking was kind of the OP's point. Put the office somewhere with actually decent transit access, and build no parking at all.
- Skyscrapers work fine for big finance companies even more paranoid about their secrets than Apple and Facebook.
- Plenty of companies larger, smaller or of comparable size to Facebook find that a skyscraper in a real city makes sense.
I think the real answer to the OP's question is mainly that the extreme power of NIMBYs in California makes it nearly impossible to build new tall buildings, so truly urban office space is in very short supply. Indeed, it can be difficult to build much of anything in California that isn't more or less in the middle of nowhere -- Facebook and Google are both practically in the Bay! The development of SoMa may prove something of an exception to this, and it will be interesting to see if many tech companies move into the new office space planned there.
If you think the world is too crowded to let existing people have a few extra years, then you must REALLY hate people with the gall to bring NEW people in the world who each expect to live in it for decades.