You actually don't want a real programming language because their unbounded loops and standard libraries are sources of non-determinism that can introduce incorrectness in your build. Bazel's correctness is highly contingent on the same set of inputs producing the same outputs, and granularly tracking dependencies, running commands in sandboxes AND having a restricted build language is a key part of it.

That said, Starlark is way closer to a real language (Python) than YAML.