For me, it would probably have to be my Bialetti moka pot and Dualit toaster. I make a pot of coffee and toast almost every morning. I also hope to replace my old pans with a cast iron one soon, and I look forward to using that a lot.
I don’t think they’re that uncommon in the US. They’re not ubiquitous like they are in Europe but I feel like the majority of people have them. As mentioned, it’s probably more to do with whether you drink tea than anything else, and not as many people drink tea all the time here. Come to think of it, I’ve never seen anyone boil water for tea on the stove in the last decade. They either use a kettle or they don’t drink tea at all.
I didn’t realize this was a thing many American homes lack or are perceived by other countries to lack? I’m in the US and have one, my teenage stepson loves it for getting water up to temp faster for his cooking. I don’t use it that often but I think it’s worth having anyway.