A refrigerator is basically a heat pump designed to achieve a temperature close to 0 °C. There are a couple of problems here:
Most refrigerators are way too weak to effectively tackle the heat output of modern PCs. It will not magically make your PC 6 °C - instead the PC will de-cool all your groceries.
Condensation. Computers and electronics do not like low temperatures because of moisture building up. Some manufacturers even warn from using your equipment before it has warmed up to room temperature.
There are better ways to remove heat from hot pieces, like biiig heat sinks, water blocks and so on. A refrigerator basically uses air, so it's the least efficient method.
(If you connected the refrigerator coolant pipes directly to hot spots of the device it would be a different thing, but simpler alternatives, like water, exist.)
