Also, none of those places are right next to a Caltrain station so the company would have to have buses shuttling people back and forth at much worse fuel economy than shuttling them from SF.
Google does have a SF office but as mentioned in other comments, real estate in SF is not cheap and it would be a fool's errand to uproar the campus in MTV.
So I think that more people taking Caltrain would make Caltrain more useful, not less.
If Muni, or SAM or VTA ran buses from SOMA/Mission to corporate campuses, those would definitely be used. But after a three-hour one-way commute that involves Muni+BART+Caltrain+city bus (an example would be a Google employee in Mountain View who lives in the Mission) even a die-hard public transport fan would switch to a company bus.
Moreover, if public transit to/from SF weren't such a fucking joke, more young techies would choose to live outside the city, knowing they could go in whenever they wanted to.
The cost and effort involved in chartering a few buses is pretty minor next to re-tooling the Muni, BART & Caltrain systems. The results are also instantaneous. They need to attract employees now, and not in 5-10 years time.
It's worth noting that a public transit commute between SF and SV can involve up to 3 distinct transportation systems. For instance, when I worked for a startup based in Menlo Park I contended with a trifecta fail-fest of Muni (bus) -> Caltrain -> local shuttle, totaling 2 hours each way.