Why do computer graphic cards use 8 pin (4 positive and 4 negative wires) connector instead of connector with only single positive and negative wire?
This allows multiple cheap connectors and wires to be used, instead of single thick wires and more expensive high current connectors. Multiple thin wires are also more flexible than thick wires. Multiple pins on the circuit board ease the problem of tracking high currents on a PCB.

It's not just about power handling, it's the voltage drop on the cable that needs to be well controlled.