system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If our heart muscles never "get tired" even if they're working hard, why aren't the rest of our muscles the same way?

I would imagine it would be a pretty good thing if we never tired out, even if it meant we weren't quite as strong, or had to compromise elsewhere.

So why aren't ALL our muscles the same way as the ones in our heart, where they'll just keep working without aches, power loss or even all-out cramps and failure when they're working hard? 

Also, I do understand that even heart muscles can break down, e.g. in a heart attack, but we never get "heart cramps" even if we exercise all our other muscles to their limits and our heart is still working normally.
gpt
Your heart muscle cells have sacrificed everything else to be good at this one task.  They can work tirelessly day after day because they’re completely gutted, removing any sort of repair or nutrition storage capacity in exchange for huge concentrations of mitochondria to drive them.  They don’t replicate themselves at any useful rate and may only divide once in your lifetime.

For a cell that’s planning to be literally in your heart and first in line for all nutrition delivery and encased in a bony armored shell this is fine.

For skeletal muscle far from the heart and more exposed to injury it’s not so good.  Storage and repair capabilities are necessary out there on the front lines, as is the ability to instantaneously apply enormous peak power rather than a more continuous pulse.