As a little kid, I never understood why my parents would abandon money at the table at restaurants when we left. (Aka tipping a waiter) Reddit: What simple concepts couldn't you wrap your head around as a kid?

As the title suggests I didn't get the concept of tipping the waiter as a very small kid. I insisted that I would take the money rather then leaving it in a public place. I was so sad when I saw the waiter or bus boy take it. Reddit, what simple concepts couldn't you wrap your head around as a kid?