People working in restaurants in the US have a lower minimum wage than the rest of the country. So letting them do their work for like $3/hr doesnt really cut it
Also, quite a few jurisdictions do not have tipped minimum wages anymore.
When a person decides to become a waiter versus a cook or a janitor, they are including potential income due to tips and lack of tips in their calculations. Thereby, the potential of tips always reduces the need for an employer to pay more.