You can extend the idle time to several hours by crossing the "Remember me?" box before logging in: then you won't be auto logged off (it's not something to be recommended though unless you really need it - refreshing some page every ten minutes or working with two windows so you can use one to 'go walkabout' is a better method).

I'd hazard the guess that the reason is not to have the servers overloaded. The more users that are tracked the more it weighs on the server, even if some of them are idle the mechanism has to keep updating who's online and what they did last.