Why Power Plant Capacity Rated in MW and not in MVA?

For the following reasons, a Power plant capacity rating may be expressed in MW instead of MVA.
In a Generating station, the prime mover (Turbine) generates only and only active power. That’s why we rated a power plant capacity in MW instead of MVA. Its mean no matter how large your generator is, but it depends on the capacity of the  engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine (Turbine rating may be seen by nameplate rating which is in MW or Horsepower (HP) not in MVA) and not by the alternator set coupled to it.

MVA is the apparant power, MW is the real power and, MVAR is reactive power. MW describes the actual power that can be supplied to the load and MVA includes both the power supplied to the load AND the power that recirculates between the power plant and the load (volts*amps).
Watt : Is For Real Power
Real Power = apparent power * Power Factor (P.F)
And Power Plants have resistive Loads So it’s rated in MW.

MW describes the actual power that can be supplied to the load (voltsampscos theta).
MVA includes both the power supplied to the load AND the power that recirculates between the power plant and the load (volts*amps).
Because power plants have to supply both reactive and resistive loads, the two ratings describe a different capability. Most importantly, all of the power electronic devices, transformers, and switchgear in the system have to be sized against the MVA rating, not the MW rating. This is because reactive loads will cause currents to flow that circulate between phases (as opposed to active loads, which consume power from the utility) but don’t actually deliver active power to the load.