We have them in kitchens that need to serve a large number of people - big offices, big hotel breakfast areas, transport lounges, etc.

But a standard kitchen, I think it's like someone else said in this thread: The time it takes to boil a 240V kettle isn't much more than the time it takes to get the mug ready, so there's no real benefit to going through the extra structural work to fit a boiling water tap.

Also I think most "boiling water" taps are actually like 95°C, not boiling, so if you're a black tea snob that isn't acceptable.

The first one I've seen that actually provides the water at a high enough temperature to make tea immediately from the tap is the quooker. I have a different one (quooker isn't available in the US 😢), but it maxes out at about 85°C, so for black tea (which is what I tend to drink) I still have to put it in the kettle to get the rest of the way. For most people in countries with 240V electricity, the difference compared to just using the kettle isn't really worth it.