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 system with 0 users, it's pretty easy to build that.
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.
Most banks and larger enterprises do exactly this. Devs don't get to go out and pick random libraries with out a code review and then it's placed on a local repository.
There are just far too many insecure and 'typo' malware to pull off the internet raw.
This is npm with all dependencies stored in a directory. Check them in. You do code review your dependencies right? Everywhere I’ve worked in the last 10 years has required this. There is no fetching of dependencies in builds. Granted, this is harder to pull off if your devs are developing on a totally different cpu architecture than production (fuck you apple).
