Why do you have to say “thank you”

Every Wednesday, my parents would buy my brother and I fast food. Recently, they decided to take it away. On the way home from church, my brother asked for it and was irritated when the reply was “no”. After he had left the car, my dad said that he took the privilege because we “didn’t appreciate it” and we were “ungrateful”. Apparently we were “unappreciative“ because we never said “thank you”.

I still don’t understand why you have to say “thank you” in order for someone to know that you appreciated the gesture. For example, if someone buys me new colored pencils and I’m super happy when I get them and I’m always using them, isn’t it obvious that I appreciated it. If I didn’t, I probably wouldn’t have been happy when you gave them to me. I feel like always expecting a ”thank you” makes your actions disingenuous because your doing it, expecting something back. Almost like a pat on the back for being nice, which is something you should be anyway. Anyone else feel this way? People are constantly mad at me for not saying ”thank you”, as if that means that I didn’t appreciate it or I wasn’t grateful for it.