Is it? Why is that?

First because you don't really have completion or templating over its content: you mostly do copy/pasting or guessing what you have to put in there.

Second because they are using all kind of tricks to add variables into it (ENV, ...): the configuration is often dynamic by nature or by re-use.

Third because it's only strings, which is prone to mistakes: they are having a huge lot of code to check those configurations.