Wire has resistance to it. The more power you try to push through it, the bigger the voltage drop, and the wire will heat up. Copper isn't cheap and the connections are limited and also have a slight resistance between the contacts. Don't exceed their recommendations. It may be expensive to increase the gauge of the wire, and the connectors available may not support it, either so that becomes a mute point.
The interlock signals don't typically have large gauge wires, and if you extend the cable too far, their level may drop as well. A square wave does not travel well without a lot of work, and the EVSE isn't designed to drive it all that far, either. This would affect the vehicle's ability to ascertain what's available and to operate correctly.
