Why is langcode in configuration files ignored during installation?
It has to work like this. The alternative would be that a single-language non-English site would need to use the configuration translation to translate its configuration.

For the en langcode configuration, the installer will attempt to override it with the provided locale translations, from localize.drupal.org or local, and store the original English configurations separately, as far as I know. So, if you install in a language where you have translations, it should use them; if you don't, you have to change your configuration to the desired language.