This may surprise you, but in some other cultures, tipping is entirely optional as we ensure our employers pay a reasonable wage to their staff.
If I go and buy groceries at the supermarket, and the cashier is friendly and helpful, I don't pay extra on my grocery bill, or slip them a fiver. If I buy new tyres for my car, and get good service, I give them my repeat busines, not a gratuity.
Why should restaurants be any different?
That said, it is not unusual to tip wait staff when the food and service is good. So I guess that our conventions don't make sense either...