When I was a server I would work hard to make sure I got every dollar. The tip is always on your mind, all night.
The other good thing about tipping is that it also makes the job a sales job. The higher the bill, the higher the tip and therefore it properly aligns incentives with the restaurant. The exception to that is fraud where a server provides free items to attempt to get a larger tip.
I get that it must be extremely irritating for people from outside the US to get a price and then have to do the math to figure out what it really costs, but that is a small price to pay for superior service all the time.