Anywhere that has any air in it will also have moisture in it, the receiver dryer is full of dessicant gel, the tiny packets you get with some new electrical items, this absorbs any moisture to protect the system from corrosion esp the pump, these ideally should be replaced every 2 years or so, which of course means opening the system so just a re pressure every 2 years will keep it in tip top condition, with a new receiver drier.
The reason why all cars need repressurising is because the atoms that make up the refrigerant(gas)R134, are smaller than the molecules that the flexy rubber pipes are made from that go from body to engine/pump, that's the same reason why fridges never need topping up, they don't have a controlled leak all the time as all pipes etc are metal.
tabetha