Headphones are basically tiny speakers. A speaker works by having electric current vibrate a magnet attached to a diaphragm, and these vibrations are the sound you hear. In wired headphones the electric current goes straight from the device to the magnet over the wire. With wireless, the headphones are basically little computers with a tiny Bluetooth chip. The device sends a digital signal which the Bluetooth chip receives. Then the chip translates this into the current that drives the magnet. So the chip needs battery power to operate the tiny computer and generate the current. But with wired, since the current is coming over the wire and there’s no tiny computer to power, no charging is needed.