Build systems are sort of like type expressions, templates, or constant expressions in a programming language. Either a program compiles or it doesn’t. What might happen when you change the code in some unlikely way isn’t immediately relevant to whether the program works now, so it’s easy to skimp on that kind of checking until things get out of hand due to greater scale.

Also, in Starlark, any runtime check you write is a build-time check and calling fail reports a build-time error, which is good enough for users, but not for understanding how to use Starlark functions within Starlark code.