Like most people, all day long, every day, you use your muscles. They don’t get stronger, they don’t get healthier, they don’t grow, they stay the same – weak.
You quit using your muscles before you reach the moment where the stress causes them to start growing and get stronger. It’s the natural thing to do, because an exhausted muscle feels unsafe and it hurts.
You need to do a many repetitions first for no reason other than to tire out your muscle, so that the last few repetitions will make that muscle so tired and that will cause that muscle to grow and get stronger.
People who weigh-train successfully pay their dues in the first many repetitions with heavy enough weight and then get all the benefits at the very end, where it gets very hard.
Unsuccessful people pay exactly the same dues, but stop few repetitions too early.