On this topic I've been wondering why are the air-air heatpumps significantly cheaper than air-water units?

As far as I could tell, there's not that much of a difference (different heat-exchanger, circulation pump instead of a fan, maybe a three way valve for hot water)- but I have hard time seeing how they contribute to the price difference of several thousand EUR.

There are also quite a few conversion projects (where an air-air unit is converted to air-water), so I'm wondering if it really comes down to just higher demand for air-air?