As noted in many other posts, tipping here in the U.S. seems to be more common than in the UK (and Europe as well). It's been noted that in larger groups, some, if not many, of the diners will 'forget' (read: intentionally avoid) tipping since it could easily be overlooked.
Point is, many of the wait people depend on tips to maintain a reasonable income for their service. Most are paid aminimum wage by the restaurant but make a fairly good overall income due to the tiipping. To assure adequate remuneration for good service the restaurant iwll add a service charge for larger groups.
Additionally, many establishments require tips to be shared among the employees... including the cooks or chefs...