Calcium and magnesium hardness is the usual cause. Cleaning removes existing scale; a real ion-exchange softener addresses the hardness that keeps creating it.
Why hard water leaves white deposits
USGS defines hardness primarily as dissolved calcium and magnesium. When hard water evaporates or is heated, mineral deposits remain on glass, faucets and appliances.
Cleaning is not treatment
Acidic cleaners such as vinegar can dissolve existing carbonate scale on compatible surfaces. That does not change the incoming water hardness.
True softening vs scale conditioning
Ion-exchange softeners remove hardness ions. Salt-free conditioners aim to alter scale behavior but do not produce the same soft-water chemistry.
Apartment vs homeowner options
Homeowners can install a central softener sized to hardness and flow. Renters generally cannot truly soften the whole apartment without landlord involvement.