This is misleading; empirically, nixpkgs is about 99% [0] reproducible already. We know that the main variance is between language-specific behaviors; Python, Rust, and C all are prone to reproducibility problems.
In general, we want the output to depend on the system architecture and the contents of URLs. Nix uses hashes to require that URL contents don't change over time, which protects from those contents changing arbitrarily.
The current community around NixOS and Nixpkgs handles these issues just fine, but if 'just use Nix' was regarded as a magic bullet for reproducibility in science, I'm guessing it wouldn't work out so well.
Fortunately, "just use Nix" doesn't do much on its own. People usually want GCC or another complete C toolchain, a C standard library, etc. and this implies that they will use nixpkgs or one of its forks. If people try to "just use Nix" in anger, then they will almost certainly be funneled into using nixpkgs as a matter of practice.
The main problem with reproducibility in science is that most scientists are not actually interested in doing science. Of course software will not fix this problem.
In general, we want the output to depend on the system architecture and the contents of URLs. Nix uses hashes to require that URL contents don't change over time, which protects from those contents changing arbitrarily.
[0] https://r13y.com/