Take the standard fridge freezer - one thermostat (in the fridge half) and one compressor.
Assume expected ambient room temperature of 20C.
Fridge kept at 5C by thermostat.
Temperature difference between fridge and room is 15C.
Freezer is -20C at this room temperature.
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Now assume the sun is shining into the kitchen, or the oven is on. The room gets to 30C.
Fridge still kept at 5C by thermostat.
Temperature difference between fridge and room is now 25C, so compressor is working 1.67 times harder.
This means the freezer/ambient temperature difference must be 1.67 times larger = -36.7C. I guess that's not really a problem, but wastes electricity.
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But what if the kitchen goes down to 10C at night? Who heats their kitchen in winter at night?
Fridge still kept at 5C by thermostat.
Temperature difference between fridge and room is now 5C, so compressor is working 0.33 times harder than the original 20C room.
This means the freezer/ambient temperature difference must be 0.33 times larger = -3C. Not acceptable for frozen food. How do the manufacturers get away with this?