There's a confusion between build and runtime dependencies. From Nix point of view. Even if you compile statically your code still depends on libc, it's just that you include the libc in the resulting binary. With Nix all dependencies are listed for your application. Because if no compiled version is available in binary cache, Nix will get a compiler and build one. This is actually awesome, imagine there's a program in nixpkgs that you want to use, but you want to have it linked with different libraries.
For example pgbouncer[1] can be linked with different DNS resolvers, which have their strengths and weaknesses (currently it is linked with c-ares which probably is what most people want, but until version 1.10 that wasn't true) you can override the derivation and use different dependency. Nix will build on the fly the new version that does do what you want, and this is awesome.
