I put the question to Mrs Zero a little while ago because she works at a plant hire company that hires generators, which are quoted in kVA not kW. Although she couldn't tell me why they weren't the same, she could tell me that they weren't, and there was a formula for getting kW from kVA. I've just been spurred on to find out why, and I have an answer, at least for transformers.
https://www.electricaltechnology.org/2012/02/why-transformer-rating-in-kva-not-in-kw.html
What it boils down to is that a transformer converts an input voltage and current into an output voltage and current depending on the ratio of primary and secondary windings - that's the VA bit as the product of the input V and A will always equal the product of the output V and A. However, the power output from the transformer will be dependent on the power factor of the load.