Been a couple years since I worked at Google, but when I was there, Stubby was pretty intimately connected with Google's networking fabric, datacenter hardware, and internal security & auditing needs. None of this is at all useful to external customers - you're not running on Google's proprietary hardware, you don't interface with their monitoring & auditing systems, etc.

As an ex-Googler, using gRPC feels just like using Stubby: the interfaces are the same, the serialization code is the same, the only thing different is the networking code and transparent hooks into other systems.