Because of all the people not logging out for days at a time, not that many other people can gain access to the game on those worlds.
SE could perform a server reboot every 5-7 hours to clean up the servers and give everybody a chance to play. I understand that reboots cause stress to the system. I also know how it causes stress when you can't log into the game because is logged in while at work/school/shopping etc. I propose the variable reboot time because if you made it fixed, some people would find a way to rearrange their schedule to make sure they would be able to log in when the server rebooted.

Just my 2gil