Because waiters in the restaurant industry generally earn low incomes, and even some waiters in some restaurants depend on tips for most of their income, paying a little extra to the waiters who serve customers to raise their income levels has become a reflection of social equality and is generally accepted by society. The behavior is generally accepted by the society.
Many people think that the tips they pay go into the pockets of the waiters who serve them directly, but this is actually a misunderstanding, because most restaurants in the U.S. have a tip redistribution process. If a table spends $200, a 20% tip is $40. This $40 has to be divided proportionally among the other service staff, for example $6-8 has to be divided among the direct running assistants; $3-4 to the barman; $2-3 to the ushers; $2-3 to the ushers, and so on. So at the end of the day, the waiter at the front desk can get his or her own tips in the neighborhood of $20, which is equivalent to 10% of the guest's spending. And the IRS, the U.S. tax agency, counts 8% of the guest's spending as taxable income for the waiter.
So, if a guest who spends $200 on a table leaves a tip of 10 percent ($20), the waiter not only doesn't get anything, but also loses money. So, tipping to 15% or more is a must. Otherwise, the embarrassed waiter will inevitably give the stingy guest a hard time.