It's that they're not economical to replace baseload generation. Those four hours they provide are during the time when demand most outstrips supply, i.e. after

It's that they're not economical to replace baseload generation. Those four hours they provide are during the time when demand most outstrips supply, i.e. after solar stops generating but before the load trails off for the evening. The price per kWh is highest then, which is what makes battery storage economical then but not necessarily at other times of day.