Why ups rating is in kva?

The kva rating includes the power factor of the connected load.

KW * Power Factor = kva.

The above answer is incorrect, as it's actually kV.A x power factor = kW.

The answer is straightforward, the output of the UPS is the product of its rated output voltage and its rated output current, and the product of voltage and current is 'apparent power', expressed in volt amperes.