Why we are rating alternater or primover in MW not in MVA and transformer in MVA not in kW?

Alternator/Prime-mover is a rotating Equipment.As it is rotating there must be so losses.So we have to consider the leakage reactance between Rotating & Stationary, there comes the Phase displace between Voltage & Current/ Power-factor.So in case of Alternator we can say Power =Rt3 X Volt X Current X Power-factor.

But in Transformer there is no rotating equipment, all are static.Power Received by Primary side transfers it into secondary side equally.There is no phase displacement or Power-factor.So we consider it as Power(P) = Rt3 X Volt X Current. no power-factor.

In alternater, mechanical power is converted to electrical power, and the reactive power can be changed at the same load by using capacitors or transformers tap changers, but MW is constant for same load.  

In transformer, Apparent power MVA depends on loads which have active and reactive power.