I wish tools like CMake and Bazel simply used Lua, Python or JavaScript for their configuration instead of making their own languages with numerous quirks. There's literally no benefit of doing that.
Starlark is a subset of Python that's only primitive recursive. And in a distributed build system, being able to guarantee termination is a very good thing. It also means Bazel can do predictable fan-outs of builds. Starlark has very good reasons for existing.
CMake almost ended up using Tcl, but it was rejected because that would've introduced an external dependency, running counter to what they were trying to achieve at the time. Would Lua have been a good alternative to creating their own (rather janky) language? Sure, but Lua had very little mindshare in 2000. Even with the weird configuration language it has, it's still preferable to the horrible agglomeration of m4 macros that is Autotools.
If you want to pick on any build system for not picking an existing language, Meson would be a better target, as it's similar to but not Lua, and a suitable subset of Starlark-esque subset of Lua would've been useful.
Lua, by virtue of being thread oblivious - may work, but under the curtain (calling "C" code) there is nothing to protect you against thread-safety related issues.
Python "deals" with it
All three options though are full blown Turing-complete languages - e.g. they can loop forever. You don't want that in CI, or a build system.
Starlark is concurrency safe. Top-level global values, once initialized, are frozen (read-only), hence they can be safely accessed by multiple threads. There are no "global" effects (AFAIK), apart from actually doing I/O by calling actions (processes, etc.)
Blaze (bazel's parent) used Python, and had these non-hermetic issues, because you can do anything with Python (actually "Lua" might be easier to sandbox, but maybe python too - not sure).
Point is, starlark is well suited for this job. It wasn't - "Hey let's design this new language". It's really Python but with limited powers for a reason, to enable other unlimited powers (concurrency, avoid recursion, etc).
