The server doesn't demand it. No person 'demands' it. It's the way the service industry works.
Chances are your imaginary server is not working forty hours a week, nor five days a week, nor eight hours a day. For VERY FEW people is restaurant service a full-time, permanent position. And you do NOT want a server with fifteen tables. FIFTEEN table? Really??? Six tables wouldn't be a small restaurant, it would be a typical (chain) restaurant with six times x tables and ideally enough servers to staff those tables.
Let's say they work the more likely six hours a day, three days a week. Out of those six hours, maybe four are spent serving. An hour is devoted to set-up and prep; another hour to clean-up and prep for the next day. So, four hours times six tables times three diners.
The economy stinks right now (not sure if you're aware) so the average check would tend to be more in the range of $15 - assuming all diners are adults. So, $45 per table, times 15%. Figure each table turns over once an hour, so twenty-four parties, or $162. The server needs to tip out the bartender (yes, even just for pouring soda), the bus persons, the food runners, sometimes the host/esses. Figure $150, times three days = $450. Add that to the - shocking! - $42.35 the restaurant PAYS them for those same eighteen hours, you get $492.35.
You end up with a much less enviable - but far more realistic - PRE-tax annual earning of $25,599.60.