Because the pulsed saw tooth wave form voltage generated via inductance doesn't produce enough voltage to power or charge. It merely produces a signal for reference. (Generally speaking) Unless of course, you need your head lamp to be the same wattage as that flashlight. Note that in those "shake and bake flashlights, you're charging a capacitor for a 5-7 minute charge on a very small bulb.
The advent of Li-Po batteries and LED lamps makes a simple battery and wall charger system the way to go for those who don't have a charging coil equipped engine, IMO.
K.I.S.S. These are mini bikes. Otherwise, a GM single wire alternator will give you 30 amps of dynamite with a laser beam, lunar eclipse, daylight and possibly enough A/H to listen to an entire Queen album on the 8 Track player mounted beneath the seat.