The real value in going out to eat, in roughly decreasing order of importance:
- The quality of the actual food and drinks that you consume.
- The convenience of not having to spend the time and effort to shop, cook, and clean.
- The atmosphere and joy of "going out" (with your friends).
- The fact that someone takes your order and brings your food to you.
There's no way that the fact that someone takes your order and brings your food to you consists of 1/5th of the value of your experience. An efficient market should then adjust to this by offering the option of not tipping, having all the consumers go to that restaurant because it's a better value, and then the other restaurants would have to adjust.
Unfortunately, this doesn't happen, probably for a variety of reasons.
1) People don't have enough information when choosing where to eat.
2) There are biases where people don't really consider the cost of tipping when they consider the cost of going out to eat.
3) People have gotten used to it, and "don't mind" tipping.