Then why aren’t they in 2/3 of houses
The temp for water used in radiators 60-70C is easily achievable by an air-top-water heat pump. It does not depend on the energy source, gas/oil/electricity.
If the water returning to the boiler isn't below 54C then there will be no condensing at all, and the advertised 90%+ efficiency won't happen till the return value is more like 46C.
That translates roughly to max winter temp of 65C leaving the boiler and lower when lesss heating is required.
This can be tweaked by the end user and save 10-20% on heating bills.
In older homes there isn't necessarily HVAC at all and instead there are actual radiators. I've lived in two like that, there is just no forced air to rooms.