Earlier quoted context omitted.
If you don't network isolate your build tooling then how do you have any confidence that your inputs are what you believe them to be? I run my build tools in a network namespace with no connection to the outside world. The dependencies are whatever I explicitly checked into the repo or otherwise placed within the directory tree.
You don't have any confidence beyond what lockfiles give you (which is to say the npm postinstall scripts could be very impure, non-hermetic, and output random strings). But if you require users to vendor all their dependencies, fully isolate all network traffic during build, be perfectly pure and reproducible and hermetic, presumably use nix/bazel/etc... well, you won't have any users. If you want a perfectly secure…
I'm not suggesting that a commercial service should require this. You asked "In what world do you have ..." and I'm pointing out that it's actually a fairly common practice. Particularly in any security conscious environment.
Anyone not doing it is cutting corners to save time, which to be clear isn't always a bad thing. There's nothing wrong if my small personal website doesn't have a network isolated fully reproducible build. On the other hand, any widely distributed binaries definitely should.
For example, I fully expect that my bank uses network isolated builds for their website. They are an absolutely massive target after all.