Dehydration and Thirst

A children’s book someone just gave me refers to the fact that we get thirsty after we are already dehydrated as a “design flaw in the human body.” Of course, I plan to color the offending sentence with permanent marker, but this raised a question. I know that Hashem created everything to be perfect, so why is it that we do not in fact get thirsty until we are already lacking in water? Wouldn’t it make more sense to feel thirst before we dehydrate?