You'll find radiators in many older buildings (before about 1930 or so, at least in my area). Some are steam and some are hot water. The problem with them is that they tend to have higher maintenance costs plus if you wanted AC, you'd need to have a separate system, including ductwork. AC is relatively cheap to add to a house with forced air heating and quite expensive to add to a house with radiators. Mini Split AC systems are changing that somewhat, but they haven't caught on too well here.

You can also find baseboard heating, but that tends to be electric and have quite expensive operating costs. It's mostly only used in additions and places where you would only heat the space if you were currently in the room.

> This is the detachable showerhead thing all over again.

Uhhh... Everywhere I've lived in the US has had a detachable showerhead, from places built in the 1890s to places built in the 2000s. It's a normal thing for apartment dwellers to buy a nicer showerhead to replace the likely cheap and crappy one that landlord installed and then put the crappy one back before moving out.

It’s because most AC systems require forced air, so hydronic is a second system.

You’ll find hydronics in expensive houses that can justify the double expense or in colder climates where the heating benefits are more important than AC.

Also everyplace I’ve lived in the last 25 years has had detachable showerheads.

i would go as far as to say that nobody in continental europe knows the term 'hydronic', it's just heating, maybe central heating.