I suspect it's because there are a lot of people contributing to the open source project - the Raspbian image will tap into that, so there will probably always be updates.
I'm very glad that a new image has been posted on the Download page. Although, yes, I can just type in all the commands to keep updated, as I said it gets to be a very boring process waiting for the (increasingly large) updates when you're re-installing your SD card several times a day trying to get that all-important completely-working base image for your hardware!
My Raspberry Pi blog with all my latest projects and links to articles
http://raspberrypipod.blogspot.com. +++ Current project: PiPodTricorder - lots of sensors, lots of mini-displays, breadboarding, bit of programming.