Wait... Google provides your cellphone directly? How did you get on their corporate plan?
Or are you talking about those Nexuses that are provided by a different carrier, and as such that carrier retains the right to do whatever they want to the OSS Android underneath?
You know... because Google can't just go on to the Verizon/T-Mobile/AT&T/Sprint network and update everyone's phone. The provider provides the specific Android build.
And that's why they can't update all the devices at once. Because everyone and their mother can develop their own kernel, and their own Android for their platform.
Now, if everyone just ran AOSP, then Google would be fine to update everyone at the same time.