Nginx was built with 'single server' architecture in mind while Google generally operates on a 'distributed everything' architecture. This involves stuff like distributed file systems/data stores. My guess is it's not compatible with the single file system interfaces of Nginx. Also note that dl.google.com serves stuff on a massively bigger scale than the majority of the rest of the internet. On that scale I'd guess they face some rather uncommon challenges and bottlenecks that can only be addressed by custom software.