Likely because, despite looking similar, those key/value and .ini style config files like every other system tool are actually hundreds of subtly incompatible or poorly specified formats. Unicode handling, whitespace handling, quoting, indentation, multi-line, line continuation, comments, non-string datatypes, lists, maps... you use YAML for human maintained and YAML or JSON for machine so people can use an off the shelf parser instead of implementing your particular key/value specification. Even TOML is better than rolling your own, since at least it has a common specification.
I once found a .cfg file. It had an incomprehensibly foreign configuration format that is specific to the file. A standardized config format like YAML or JSON has lots gotchas but you can learn them once and then know forever. With custom_format5345 you have the problems of poorly designed configuration formats (lots of gotchas, sometimes more than YAML) and the downsides of having to relearn the undocumented configuration format.