If your config language doesn't have its own control flow then it's going to get a meta layer added on with control flow. Like, what do you do if this service has a different hostname in staging versus prod? Or connects to a different DB, or whatever. Either have one template file and give it a values file when you go to deploy it, or have two fully-written out files... which becomes annoying when you have to make sure you keep them in sync across two or more environments, often leading to a hacked on and poorly implemented templating system anyway (eg, Helm)
If your program requires executing another program to prepare its options, then something has gone wrong in design or architecture.
Configuration is a type of API - and should be designed with the same care and goals including human legibility, orthogonality, “simple things should be simple to do”, surprise avoidance, stability, minimalism, etc.
Instead often what we get is little more than a sort of serialized form of the internal state of the system.
