Generators and alternators are rated in kVA because they generate both active and reactive power which is rated in volt-amperes (VA) instead of watts (W).
The main factors manufacturers consider while designing electrical devices and appliances which provide electric power like transformer, UPS, alternators and generators etc, are load and power factor.
As they don’t know exactly what is the power factor and which kind of load (resistive, inductive, capacitive or a mixture of all of them) will be connected to the device and appliances.
So they simply design and rate the electrical device according to its maximum current output that the conductors can safely carry while they consider unity power factor (In case of pure resistive load).
For this reason apparent power measured in kVA is regarded as the rated power of the alternator¹⁵.
On the other hand, motors are rated in kW because they consume only active power in watts as input and provide mechanical power in HP or kW at motor shaft as output.