I guess everyone is talking Of the order 80 Amps and higher without thinking. A casual observation of cables that carry the currents are not thick to carry such high currents and such high current handling is a safety hazard.. Also, given the fact that cars move and hence the vibration will cause heavy sparking and damage the wiring.
Only the cable from battery to the starter motor is thick as it carries a peak current of upto 400 amps At engine cranking moment but then to a fraction of a second and not continuously!