Seems like it could be rather expensive to retrofit, compared to cost of putting it in the house to start with (certainly I couldn't imagine getting it retrofit

Seems like it could be rather expensive to retrofit, compared to cost of putting it in the house to start with (certainly I couldn't imagine getting it retrofitted into my UK house -- the labour alone to dig channels into walls and then make good again and redecorate would dwarf the equipment cost). But the thing about features put in a house when it's built is that they have to meet the economic bar of "do potential buyers value them enough to pay XYZ more than they otherwise would", which can be quite hard. If the average buyer wouldn't value the house at an extra $1000 then it's not worth the builder installing one...