I think two things. One is just size. The other is complexity. A fridge uses a few hundred watts. A HVAC heat pump uses a couple thousand watts. A fridge has

I think two things. One is just size. The other is complexity. A fridge uses a few hundred watts. A HVAC heat pump uses a couple thousand watts. A fridge has a simple hermetically sealed compressor. With a condenser and evaporator that are just long tubes. All the connections are brazed at the factory. Dirt simple. A heat pump has a external motor driving a compressor. So you need seals on the shaft. And then the condenser and evaporator are more complex. Some designs need blower to blow air over the evaporator. And because it's reversible you need valves as well. More complex.