Because it wouldn’t be effective. Either you’re running new water from the tap through the radiator, which is expensive and wasteful. Or you’re sending the warmed water back into underground pipes to cool it off again. If you’re going spend a bunch of money to bury a bunch of pipes, it’s better to something that works instead.
A ground source heat pump is an effective way of getting the cold from the ground into your living space. A ground source heat pump is basically an air conditioner that has the hot part buried in the ground, and they are very efficient.
Because it uses refridgeration, its far more effective and efficient than backwards radiators.