Why do we have to restart routers?

jaypaulw writes: "I've owned a WRT54G, some cheap D-Link home wifi/firewall/router, others, and now a Apple Airport Extreme (100/10 ethernet ports.) In the context of the discussion about the worst places Windows is installed (in places where an embedded device is superior) I've gotten to wondering why rebooting these devices so frequently (often every few days) is necessary. It seems like these (purpose built with an embedded OS) should be the most stable devices on my network."