Why transformers rated output expressed in kVA and not in Kw

Transformer rating is in kVA not in kw

The manufacturer of transformers fixed a name plate on the transformer on which are recorded the rated output, the rated
voltages, the rated frequency etc. of a particular transformer.

A typical name plate rating of a single phase transformer is as follows : 20 kVA, 3300/220 V, 50Hz. Here 20 kVA is the rated output at the secondary terminals.

Note that the rated output is expressed in kilo-volt-amperes (kVA) rather than in kilowatts (kw). This is due to the fact that rated transformer output is limited by heating and hence by the losses in the transformer.
These losses depend on transformer voltage (core loss) and current and are almost unaffected  by the load power factor.

Consequently the transformer rated output is expressed in kVA and not in Kw.