Because doing it that way will make building (and verifying) a deterministic build more difficult for users, while forcing the builds to be deterministic in the

Because doing it that way will make building (and verifying) a deterministic build more difficult for users, while forcing the builds to be deterministic in the face of randomised non-determinism means that anyone can build the project and get the same output without needing any complicated build configuration. That end goal (all builds are deterministic even if you don't have some magical reproducible build machine) is the holy grail of reproducible builds. And since this is run as part of a CI process, you will get lots of builds over time and will root out all sorts of issues caused by non-determinism.