Then it sounds like you don't actually believe you're at 0% remaining. What your parent is saying, and what I've observed myself, is that most people really believe they physically cannot (say) run any further.
Do they? I would expect that when you "physically cannot run", it means you are collapsed in a pile on the ground unable to stand or move. People might say things like "I cannot run another step", but that is not the same as actually believing it. That said, unless you are very fit and very young, it might not be a good idea to get yourself to the point where you actually cannot run anymore. Unless there is a tiger chasing you or something.
This https://www.youtube.com/watch?v=CKWWRS9CpTY is being at your limits. But I'm willing to bet that a tiger would make her body remember that maaaybe there is actually (a lot) more energy left to keep going.
Of course. When I do my reps in the gym, I know I could do more if someone put a gun to my head. I stop because I don't want to hurt myself, not because I cannot do anymore. When I feel sleepy in the evening, I am fully aware that I could go 48 more hours without sleep if I really needed to. But would it be a good idea? No, it would not be.
And even if we ignore obvious health risk of pushing yourself to the limit, in general, I would rather be more comfortable than more productive. There are always things to be done, so why stress about doing a little bit more?