Nuts. If someone is at a restaurant that serves people at tables/etc., they receive excellent service, the server was polite, and they found no reason to complain about out loud, then a tip is almost necessary. "Tip" actually comes from an acronym that means "to ensure promptness." If you don't pay, then you won't be served 'til doomsday, armageddon, or ragnorak.
Rubbish.
A tip is a kindly gift that means that the customer enjoyed the food, service, and everything else and thought of adding something to the server's paycheck. Fifteen percent is for an okay, good, or just plain tasty meal, twenty percent is the equivalent of saying "I loved the place and I will eat here for many times to come!" Higher is plain ridiculous, but if the restaurant was REALLY TRULY ABSOLUTELY FANTASTIC, then you could leav twenty-five or thirty percent.
Some people are just in the habit of not leaving a tip. Leaving something for good service is not prevelant in other countries (aren't you supposed to give excellent service anyways???), some people might expect higher from waiters/waittresses and want them to perform higher, some people don't have the dough, moolah, bucks, iron men, dead presidents, greenbacks, bills, checks, lire, pounds, pesos, dollars, or M & Ms to tip the server, and some people are just pains in the Equus asinus. (That means donkey, by the way.)
Perhaps the money was stolen. Other pains in the Equus asinus steal, I mean STEAL, the tips from under the plates or beside the place-mat or whereever someone may put one.
Last bet: some people have NO IDEA what the heck things cost these days. When I still read Captain Underpants comics, it said about real grandparents: "...and they don't know what things cost."
"Here's a dime to buy a video game."
"Thanks. (Sigh.)"
The robot grandparents: "...and best of all, they don't know what things cost!"
"Here's $10,000 to buy a candy bar!!!"
That obviously is NOT true, but sometimes you run across a person who thinks gasoline still costs $0.95 a gallon. They may just not tip for that reason. OR they think the tip is in the paycheck, so they don't tip.
-Your Eatery Expert