I guess its because you can allow for custom data formats. You'll have to validate/parse the file anyways, and maybe having utc timestamps is worse than local date-time notation. Especially if the user is supposed to edit the file by hand.

I know for sure I'd like "timeout: 1h6m10s" more than "timeout: 3970". So unless you want to support really specific datatypes just being typeless is better. Putting everything in double quotes to get a string, while spec-wise would be typed, is not enough when the backing data type is not going to be a string. So you might as well throw it away and let the program handle all type conversion.