Why Generator & Alternator rated in kVA. Not in kW?

The power √3 VL IL Cos φ delivered by the alternator for the same value of current, depends upon p.f. (Power Factor=Cos φ) of the load. But the alternator conductors are calculated for a definite current and the insulation at magnetic system are designed for a definite voltage independent of p.f. (Cos φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternator.

Generator and transformer are rated in KVA becoz at that time we dont know the consumer power factor as we know that the (KW= KVA X Cos Phi) as we know the power factor we have ratings in KW and on the other hand motor has defined power factor and the rating has been mentioned in KW for example if we have transformer of rating 50KVA this rating of transformer is its apparent power that is KVA if we multiply the same withh the power factor 0.85 we will get the real power that is KW.
KW= KVA X Cos phi
KW = 50 X 0.85
KW= 42.5KW

All electrical devices that supply power are usually rated in kVA. This includes transformers, UPS’s & the like.
Simply put, the difference between kVA & kW is that kVA is “Apparent Power”, which does not take into account the Power Factor & kW is “True Power” . See the below equations:
VA = E x I
W = E x I x Cos phi
The above equations are only for single phase.
When a manufacturer makes a transformer, generator, UPS etc, they have no idea of the type of load that will be used & consequently can only rate the device according to its maximum current output that the conductors can safely carry (at untiy Power Factor) & the insulation rating of the conductors (voltage & temperature).