Having been here a long time, and been through countless birthday celebrations you run out of things to say. "Happy Birthday! You are a valued asset to the company." only goes so far. After a while I would add a line about their actual value, in dollars and cents. Unfortunately sometimes I had to report "you are a debit to the company, and they are thinking of cutting you loose."
A random quote generator helped for a while. "If at first you don't succeed why go on and make a fool of yourself?" "War is God's way of teaching Americans geography." But that got old.
I started making things up, figuring nobody ever read them anyway. "Happy Birthday. Without you as a coworker my life would be an endless torment, a living hell." or one of my favorites. "Happy Birthday! We all notice you've put on a little weight, that is why Mable made a fat free cake, it tastes like cardboard, only drier. But at least you might be able to button your pants tomorrow, tubby!"
For a brief time I used words cut from magazines. I would paste them on the card, and make demands for cash. Of course then I would sign someone else's name.
Now, I just pick a birthday wish that seems appropriately long, cross out the name below it and sign mine. I am thinking of asking for a raise, it isn't often an associate strives so tirelessly for efficiency.