put it this way. Microsoft don't update Windows main installers. you buy say XP, Vista or 7, and it's the first release. you then have to apply the service releases and patches.

you think lazy. but if you take a second look... it's the simplest method. you don't get confusion about what version you are using to install. everyone starts off with the same version. then, brought upto date via patches. simplest for support and logistics.