Some families don’t give money. Some people think it’s awful and shows no thought to give money. I assume children in those families never expect money in a card if they have never received money, and grow up to be teens and young adults not looking for money.
I was given money on holidays and birthdays since I was a little girl, and I never expected to get it even with that history, but it was always appreciated. I still get Birthday cards with checks from my parents.
My husband’s family doesn’t usually give money to kids, but I was the aunt who did. When his niece and nephew were very little we bought gifts, but I remember the first Christmas I put $50 cash in each of their cards that was with a gift and the same to their stepsister and stepbrother since we all were together for Christmas, and they were so excited about the money. Actually, the stepsister was the most appreciative and obviously recognized it was nice to be included equally, she was the oldest in the group, around 14 years old.