Why can’t you de-scale hot water tanks?

[Fluffycloudland77]

They must get limescale too.

mainly because the thickness of the deposit inside the tank is so great that you just can't get enough chemicals in there to remove it.

You can but you would need a lot of chemical and also ensure that it is correctly isolated and flushed before putting it back on the system.
It's probably easier to drain the tank and remove the immersion elements and chemically clean them, however they are relatively inexpensive £15-20 so easier just to replace.
Not sure on you home country rules regarding electrical work certification (part P) but legally speaking the job is outside the realms of DIY so the is where the costs build up.
Maybe someone else can give a better solution.