It's actually the combo of very little resistance and the capacitors used in the ESC circuit. This is a type of "Hard Start" system that is similar to an instant dead short. If you unplug the ESC and then plug it in shortly after the spark will be much less if nothing because the caps are still partly charged.
There are some articles that talk about using a "soft start" circuit, especially on high voltage systems to elliminate this arcing, so that your contacts don't get too corroded. They use a resistor and another lead with the negative lead in tandem to charge the caps at low current and then plug in the main positive lead for main power. I think I saw this detailed in one of the recent MA magazines.
Curtis