It’s not. The kettle attracts and holds ~~heavy metals~~ minerals and lime that would remain in the water of your glazed or glass mug after microwaving.

I've microwaved water and there is always this foam at the top. Furthermore I think the kettle takes away some of the Particles since mine always has timescales after a few days.

Using a kettle is not just a British thing, it is always a good idea to boil water, let it cool down and then drink, I recently read that it also reduces micro plastics by at least %70