A larger reason (though not nearly the only one) was Beverly Hills lobbying against the influx of homeless people it would be bringing in. Not to mention all the permits, beaurocracy and difficulty of burrowing beneath an active city. But sure! Whatever the reason, trains aren't gonna extend themselves in the time I can hail a very real Waymo.
I'm not saying it wouldn't be nice. I bike or take the train to any destination that is practical to do so. But what isn't practical is screaming "fuck cars," especially Waymo -- an electric car that takes me safely and directly to my destination -- in a city where the only other options are pies in skies.
Because the argument is specifically driverless cars vs trains