That’s exactly the point. Home connections are limited to a specific amperage (15 or 16) pretty much all across the world - certainly not beyond that anywhere. Voltage remains the only difference: 120 vs 240. With 120V at 15A you can only go up to 1800W which does take a long time to boil water.

Yes, you could have multiple phases to get to 240V in 120V countries to boil water faster but these would be special outlets and not meant for a kettle that you could buy from a Walmart.