Because people want to set the fridge temperature precisely to 6C, and not turn a century old tech of a mechanical thermostat located inside it to an arbitrary setting between “off” and “three”. And if you want to be able to do that, an electronic switch is literally the only way: you now have a computer that is responsible of doing “if current_temp > set_temp then compressor = on”. When that computer stops working, so does the fridge. Not an issue if that’s the only thing that computer does. Probably an issue if it’s complicated enough to support Xbox live streaming so you can play Doom on it.