Answer
Because there are already electrons in the wire; the electrons do not start at the source of power. Flipping on the switch completes the circuit causing electrons to flow into lightbulb. Again, since there are already electrons in the wire, turning on the switch just sends the closest electron in the wire to the lightbulb into the lightbulb to produce light.
Work Step by Step
Because there are already electrons in the wire; the electrons do not start at the source of power. Flipping on the switch completes the circuit causing electrons to flow into lightbulb. Again, since there are already electrons in the wire, turning on the switch just sends the closest electron in the wire to the lightbulb into the lightbulb to produce light.