Installable storage heaters - why manufacturers bother putting the bricks in?
Good observation on the relative merits of water.

However, your key assumption about relative heat capacity is almost certainly wrong: Many common metals have volumetric heat capacity exceeding half that of water.  And many ceramics have higher heat capacity than water!

You could still argue that it would be cheaper to produce and ship these storage heaters empty, and then fill them with water once they're installed.  But then you would have to consider the unique drawbacks of water:


It is corrosive to many metals
It expands when heated (and also if frozen)
It can't be heated beyond its relatively low boiling point in a non-pressurized system
If its container leaks it causes a lot of collateral damage.


Plumbers tend to view water heaters like time bombs: No matter how exotic the alloys and coatings used in their containers, the effects of expansion and contraction seem to always find a way to break them.  I.e., it's just a question of when, not if water heaters will leak.

Granted, it is possible to build water-based heat systems that last for generations without leaking or failing.  But in a smaller system it might not be – in fact, as you observe, evidently is not – cost effective.