Why does my tester not indicate voltage when a light switch is on?
If you consider this simplified diagram, showing a light &amp; switch, with the switch in the off position on the left, and in the on position on the right:







When the switch is OFF, its contacts are open - preventing current from flowing through the circuit to the light bulb.
When the switch is ON, its contacts are closed - allowing current to flow through the light bulb.  

Your "voltage tester" is measuring voltage, not current.
When the switch is on the ON/closed position, your "voltage tester" measures 0V across the switch because that is exactly the case - a closed switch is a short-circuit and even though there may well be current flowing through it, there is no voltage across it.  

If you were to put your "voltage tester" across the light bulb instead, you would find exactly the opposite case.
With the switch OFF/open there will be 0V across the light bulb and when the switch is ON/closed you will measure a voltage across the light bulb.