Nix builds go through 2 phases, first downloading the dependencies into the nix store (which can also go through a nix build) and then executing the builder with access to the nix store but without network access. This is done for purity because network access can lead to non-reproducible builds.
Most languages and tools are complex but can output URLs to their depdencies or download them, but that cannot happen during the build (no network access). So, either a tool predownloads stuff and they can be put into the nix store by the "simple" nix derivation, or the tool (pip, docker compose, cargo,...) has to be rewritten either partially or completely in nix. The latter is much harder and in most cases would just lead to repeating the work. Nix may be a functional language but it's missing important language features like static typing.