Why transformers rated in MVA not in MW?

Transformers are rated in KVA and MVA. The load determines the power factor. It is obvious to multiply the VA with cosine angle to get Watts or Kilo watts. While manufacturing the transformer or at its installation we don't know the load power factor. Hence we could not determine the rating in Watts or kilo watts or Mega watts. We simply note it as VA or KVA.

S.Dhanabal