Why Transformer Rated in kVA and not in kW?

Generally transformer rating is in kVA because “ Copper losses ( I²R) depends on Current which passing through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage. So the Cu Losses depend on the rating of current of the load so the load type will determine the powerfactor (P.F) , Thats why the rating of Transformer in kVA , Not in kW ”.