DAE never know what to put in birthday cards

i have nothing to say other than "happy birthday", but then the card itself usually features that writing in the middle already. but then i see the cards other people are giving and receiving and they often have multiple lines of writing in them, and i have no idea what you could possibly be saying.

why do people even give each other cards?? i used to have a big box of ones i received throughout my childhood, but i never looked through them until i had to get rid of them to free up space, and looking through them was pretty boring.