Because Google isn't the manufacturer of Google-branded Nexus devices and, if you bought it elsewhere, isn't the retailer, either.
> Apple manages to support iPhones whether you purchase them from an Apple store or in a back alley for bitcoins.
Apple is always the manufacturer of iPhones.
I agree that it's bad for Google's image to operate that way, stick their name on a phone and then shrug their shoulders and tell you to talk to the real manufacturer. But that distinction is why Apple supports most iPhones no questions asked, and Google wants to know that you bought it from them. They're a glorified retailer licensing out their brand to Huawei.
I'm not 100% sure, but I don't think that's how it was. LG's Nexus devices were manufactured by LG for LG, etc. The "Google" part of a Nexus phone is the software and some collaboration in the design.
If it was advertised as a Huawei 6p that would be different.
The Pixel, instead, is branded as the Google Pixel.