There are a few factors here:
Temperature range. Domestic heating water ranges from about 90 F (35 C) to 140 F (60C) The latter is used in radiators and fan coil heat exchangers, the former used in in-floor heating.
Swimming pools are heated to only to mid 80 F. Depending on how much water you move, you need water only a few degrees to maybe 15 C warmer.
Most of the time swimming pools are used only in reasonable weather. The costs to heat an outdoor pool in mid winter is prohibitive. So the heat pump isn't working with a large temperature differential.
A house heat pump has to work in mid winter. This requires different engineering.
If a swimming pool heater breaks down, it's an inconvenience. A swimming pool takes days to weeks to cool, depending mostly on evaporation. A home pool owner can decide that he can wait to fix it in a few weeks, or next spring.
A home heating system breaking down is a "We need to fix this RIGHT NOW"
Most home systems are reversible. They can be used as cooling systems during the hot part of the year.