Mostly for simplicity's sake, as well as having less information for the server to juggle. It is assumed that when you go anywhere you would refuel, no need to make the game a more tedious simulation than it already is.
There are several things that do not happen in this game because they are considered to menial.
Examples: Eating, urination, excretion, fueling, sleeping, etc.
I believe, though I'm not 100% positive, that these things are left out as to give players a sense of reality but without having a SIMS game on their hands. If you had to do those things, you would most likely have to log on at certain intervals just to make sure that your player is doing the things they are needing to do to stay alive. The design of this game is more to let players interact as their time allows them too...and not every few hours.