Why not in the US?

Because it's illegal in 49 out of 50 states (legal in Utah as April) AND there aren't any units available for sale with 120v output (and the required auto shutoff when the grid is down)

Most of the German people I've asked say it doesn't make sense to have a battery storage, but they also live in small apartments with little outdoor space for multiple solar panels. Theoretically, if there is enough space for panels a battery backup could keep supplying the max wattage (800w Germany, 1000w Utah) throughout the night