Because it's part of our culture and it's an easy way to show appreciation. You don't have to do it anywhere. Waiters also explicitly aren't allowed to be paid less. They must make at LEAST minimum wage. If they don't make minimum after tips, their hourly rate is raised each paycheck to equal minimum wage. It's literally not the customer's problem. You should probably learn how things work here if you're that curious!

You do realize that so called "tipping culture" in USA is literally racism? Are you showing appreciation to racism?