Because they didn't listen what their bodies were saying.
There is a reason why you don't want to exercise.
When you feel you are very tried, you overdo something
When it hurts, you are doing something wrong.
When it's too easy, you go a bit faster or longer.
You may push yourself somewhat, but if you have to push all the time, then you are taxing your body.
That's why I always say "it's the feeling that really matters".