See, turning on a computer is an explicit declaration that you want to use it. Windows updates ignores this declaration by taking that time to itself. It doesn't matter if I can defer the update by how many days if one day it will stop/delay me from using my computer when I want to. Almost no one outside of IT Administrators turns on computers solely for updates and maintenance.
Contrast that with Linux (Ubuntu distro in mind) where updates happen when the computer is shutting down. In contrast with turning your computer on, shutting down is a declaration that you no longer need your computer. That's the OS' cue to chime in and perform some routine maintenance.
I wonder how much of this is an architectural problem. The first time I remember being frustrated with this Windows behavior was in Vista. Someone correct me if I'm wrong but I seem to recall that 95, 98, and XP used to do updates on shut down. I know Vista isn't fondly remembered but, gee, did they deliberately make it so that it updates when you start your computer? Why couldn't a company who takes pride in backwards compatibility keep the old update behavior?
To wit, people make a big deal about how immediately can you use your computer (i.e., start-up times) but all your fancy-schmancy SSD + hyperthreading set-up is useless if Windows decides to update just before that one presentation you've spent weeks on. Updating on shut down is the user-friendly way.