American kettles are significantly worse than British kettles. They run at lower voltage and lower amperage, so they take much longer to boil water.
Given the choice between using a multipurpose microwave to do one more thing, and buying a separate appliance that is no faster, choosing to use the device you already own is entirely appropriate.
Yet another demonstration of the inferiority of the metric system. More than 90% of the US population lives within 5 kilomiles of our largest train line.