It's just a matter of buying the right equipment. If you buy the wrong inverter, it just switches off when the grid goes out. You need some device that detects that and can disconnect from the grid (to prevent power going out to the grid) and then power the house. This is called islanding. Not all inverters support this. The ones that don't will switch off when the grid goes out. In addition to that, you might need a physical switch or transferring device that takes care of making sure no power leaks to the grid in case of an outage. Setups without this require inverters with anti-islanding that will switch off if the grid power drops.

Outages are rare enough here in Germany that few people go the extra effort and cost to install the necessary equipment. But with the right equipment (which is available if you go look for it), you can definitely get this installed. In places like Australia, outages are more common and it's generally not that costly to upgrade your setup to support proper islanding and it's more common for people to design their systems for this.

Of course people if feel compelled to prepare for WW III that might be a good motivation. On the other hand, we'll have bigger issues if it comes to that.