Final answer:
The spark plug wires are thick because of the thick insulation needed to contain high voltage and prevent arcing, not because of a need to carry a high current.
Step-by-step explanation:
The spark plug wires in your car are designed to transmit very high voltage from the ignition system to the spark plugs, which ignite the fuel-air mixture in the engine's cylinders. The core reason these wires are very thick isn't due to the wire itself needing to carry a heavier current, but rather, the wire is surrounded by thick insulation. This insulation is necessary to prevent the high voltage, which can be thousands of volts, from arcing to the engine or other parts of the car, which could potentially cause damage or pose a safety risk. The actual current that flows through spark plug wires is not exceptionally high; instead, it is the voltage that is high and the insulation that prevents current from leaping off the conductor.
The wire gauge system, including American Wire Gauge (AWG), is used to standardize the sizes of wires. Spark plug wires differ from regular household wires (which typically range from 10 to 14 AWG) because they need to insulate against much higher voltages, hence the increased thickness.