The SF one would apparently be near union square. Having been there, that seems easy enough to get to, insofar as anywhere is easy to get to in SF (it does not exactly have the greatest public transport in the world).
The train stations have connections to other transit - hard to see them as any less convenient than airports (LA's famously had no train connection until recently).
A train station in downtown LA or SF would be inconveniently located for most people in these cities.