That's like saying, "why did you introduce entropy?" It's there. It happens, and it's difficult to get rid of in any complex system.
https://en.wikipedia.org/wiki/Reproducible_builds
https://wiki.debian.org/ReproducibleBuilds
https://www.win.tue.nl/~aeb/linux/hh/thompson/trust.html
Debian was one of the first large distributions to use a build farm for many different architectures. Maintaining deterministic builds (even as far as just file timestamps) in a distributed environment is very challenging.