I'd actually be interested to know how many mitochondria are there in the heart of other species when compared to humans, because the mere assertion that the heart doesn't get tired because otherwise we would die does little to explain the mechanisms behind it.

This. Beating heart on an ultrasound is widely considered the first true sign of a healthy pregnancy. Almost before anything else, the heart develops and immediately begins beating, and does so until you die.

There's a reason why the cliche X is the beating heart of Y.

> It doesn't because it can't. Any hearts that got tired died, and their owners did not reproduce.

They're asking why the heart can't get tired. You answered why we evolved such that the heart can't get tired, which is not the same question.

True. My answer was more of a cri de coeur.