Build machines generally want to be 'online' so they can connect to the internet. They want to connect to the internet so they can access services hosted on the internet. Many of those services use Cloudflare, Akamari or Github. If any of those are down, those services are down. Now it's great that your build machine is online, but the packages it's trying to fetch won't be available because their CDN is offline.
Now if only you had a local caching proxy.