The early versions (before it was called Starlark) was indeed just run with a Python interpreter. But in order to have a deterministic and reproducible build, t

The early versions (before it was called Starlark) was indeed just run with a Python interpreter. But in order to have a deterministic and reproducible build, they needed to put some limits on what you can do, and especially what you can accidentally do. Starlark's raison d'etre is to allow a build graph that you can reason about and have a shared cache for.