The question was asked: "What's the best way to explain the phrase 'leaders are born not made'?"
That's easy. The best explanation is that it’s pure bunk, not supported by any evidence whatsoever.
But if it’s bunk, why do people repeat it? Perhaps they are making an excuse for not being a leader themselves, and not doing anything about it.
If they did want to do something about it, they could find many resources to help them grow into a better leader. My own contributions include:
But I'm not the only leadership coach. You have many choices, so if you catch yourself arguing that leaders are born not made, stop making excuses.
If you want to stop making excuses and start making a leader out of yourself, find yourself a collection of coaches and get busy with the making.