We know how to make inverters that can produce an AC voltage on their own. It is not particularly difficult or expensive. The reason why we do not put those inverters in plug-in solar units is entirely because we don't want them to have that ability.
Those are a different design inverter than cannot plug into a wall. It isn't hard to design either type, but they need different designs in some critical areas. (you can design one that will do both as well, but it would be more complex)