That kind of ignores the entire pipeline involved in computing the correct config. Nobody wants to be manually writing config for dozens of services in multiple environments.

The number of configurations you need to create is multiplicative, take the number of applications, multiply by number of environments, multiply by number of complete deploys (i.e. multiple customers running multiple envs) and very quickly end up with an unmanageable number of unique configurations.

At that point you need a something at least approaching Turing completeness to correctly compute all the unique configs. Whether you decide to achieve that by embedding that computation into your application, or into a separate system that produces pure static config, is kind of academic. The complexity exists either way, and tools are needed to make it manageable.