Firstly, because if you do it well enough you’re gonna get a sick tip. I used to work in a themed restaurant that did like 13 different birthday songs depending on whether it was a child or adult”male or female, or whatever. The goal was to embarrass the birthday person as much as possible. It’s not for them. Not a single person I’ve ever encountered in my 15 years of serving has ever gone somewhere.
The number of times I’ve had a guest pull me aside privately before the meal ended and begged me to not sing to them is incalculable. And if they’re paying, I won’t. But 9/10 the person who is paying isn’t the birthday person, it’s their mom, gramma, or asshole friend who wants to laugh at their misfortune.