Two reasons: 1. Battery leakage. You can't charge a battery and leave it somewhere disconnected and expect to come back to a full charge. Small amounts of powe

Two reasons: 1. Battery leakage. You can't charge a battery and leave it somewhere disconnected and expect to come back to a full charge. Small amounts of power leak all the time. Look for "self discharge". There are lines of batteries (particularly Sanyo Enelopp) that are marketed as low self discharge i.e. they can be stored charged and will still be usable in time. There is still a high resistance when something is connected which is enough to help with self discharge. 2. Most phones have a non mechanical on/off switch so a tiny bit of current is still used to have soft on/off functionality and watchdog circuitry online.