You say that, but people in OCaml keep bemoaning the use of mostly declarative s-expressions in the Dune build system. Imagine the reaction if MSBuild used an actual Scheme.

Why doesn't the OCaml build system use OCaml?

You don’t want a language with non-determinism, arbitrary IO, impure functions etc. for build configuration ideally.

I guess the answer to your question is OCaml has unmanaged side effects.

They don’t know how good they really have it :)