Why do batteries in smoke detectors have to be replaced in 6 months?

They don't. Normally the 9 volt alkaline batteries in a smoke detector need to be replaced every year. If yours needs to be replaced every 6 months then something is wrong. If you are using a standard battery and not an alkaline battery it may only last 6 months. Use nothing but alkaline batteries. You sould hear a chirp from the detector when the battery needs replacing. A hard wired smoke detector has a battery back-up that should last around 2 years. If it only lasts 6 months then there is no power to the smoke detector and it is wired wrong or the wiring has come loose. The only reason to recommend changing batteries every 6 months is bacuase people tend to not change the batteries as they should. It has been proven by tests that about 35% of smoke detectors in homes have dead batteries.