But cars are already broken: see LA, SF, NYC. Why double down rather than invest in infrastructure?

I personally agree, but most people like cars as they are now, even in SF and LA; only in NYC is public transit really good enough that there is a critical mass of non-car owners voting to keep up the public transit.

I live in silicon valley, and the traffic isn't SF or LA bad yet, but it's pretty bad, and it's also clear that the voters around here aren't prepared to vote in usefu public transit; (one could make the argument that VTA is proof that voters are prepared to pay for public transit but aren't willing to allow useful public transit)

The upshot is that most people outside of NYC don't think automotive transportation is broken. Most people prefer automotive transportation. I personally am with you; driving when you could be on transit is nuts, but I'm clearly in the minority.

Because being easily deployable is a very important part of a technology.

Only to short term business interests. Long term the costs of cars to society are absolutely abysmal compared to bulk transit.