>Why would they be ubiquitous at the household level, not centralized as a "electric grid backup battery", like Australia's "big battery" https://victorianbigbattery.com.au/
For the US, there is little chance that such a centralized backup system would ever be built. The US doesn't really build big infrastructure things like this. They prefer everyone to have their own little version, at higher cost and less efficiency.
So about half the size of the battery in question. That is to say, Australia built a centralized battery almost twice the size. Their population is 25m. California’s is 40m.
There's a commercial incentive to build electric storage, if the price is right; you can buy when prices are low and sell when prices are high. Depending on the market regulations, prices are likely to get high during an outage, so there's your motivation for construction. Of course, siting is important too; if the big backup battery is right next to the big power plant, that helps for power plant outages, but doesn't help for wiring faults. And a battery facility that's not sized to power the grid island it finds itself in likely can't provide any power, unless the grid island has sufficient load shedding mechanisms, I'd guess.
For the US, there is little chance that such a centralized backup system would ever be built. The US doesn't really build big infrastructure things like this. They prefer everyone to have their own little version, at higher cost and less efficiency.