Wait... Google provides your cellphone directly? How did you get on their corporate plan?
Seeing that google sells phones on Google Play, don't give a crap about what carrier you have, and these phones works, I'd say your sarcasm is seriously misplaced.
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?
If you believe that there only exist phones with carrier-specific/altered OS, you're the perfect client for them. All the crap they keep doing to phones (locking them on specific networks, adding crapware, removing legitimate options/tools) is totally irrelevant regarding the ability to use a mobile phone...
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.
Let's look at the Nexus5. Google produce a firmware, google put said firmware on their servers, the phone connect to these servers, and get the update. At what point exactly is the carrier doing anything, beside *maybe* providing the data connection (supposing you're not on wifi when the phone checks his updates)?
Same thing for phones from other manufacturer; Samsung handle updates of their devices, LG does the same, etc.
Carrier are only concerned when they sell severly modified version of the OS, where they take pride to redirect the update lookup to their servers, and only provide updates really late (if at all).
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.
Quite the contrary. Google provides OTA updates progressively, probably to limit the impact of large unknown bugs. A friend got a notification way before me about his Nexus 5 getting an update. Still, if you want to update before you get the notification, you're free to do it with the exact same image provided online. The latency here is voluntary, and don't have much to do with carrier.
And about AOSP; even if everyone where using it, Google couldn't do squat about updating every devices at once *especially* because the kernel is the only thing that have to be device specific. Sure, the fact that some manufacturer likes to add their own UI and crap is also an issue, but you can't just put AOSP on any phone, you have to have a working kernel for it.
Term, holidays, term, holidays, till we leave school, and then work, work, work till we die. -- C.S. Lewis