Think about this: your microwave and a kettle can pull the same 1500 watts from the outlet. A kettle can nearly directly put that energy into the water with its heating element, heating it. The microwave turns that energy into microwaves via a magnetron that then heats the water. The microwave also has a fan to cool its electronics, lest it overheats. That’s energy that isn’t going in the water, but is enough to damage things.

Now, why do you think a similarly powered microwave could beat an electric kettle?