Because at multiple points humans are involved creating context for the values and later debugging or changing meaning of the values. Fun part it is when you integrate with systems where you even don't talk with the other party - you still need the documentation but as we see how much JSON over HTTP is used it is rather clear why we need text format.
Not sure what context creation means here that requires machines to speak text to each other, and you could debug the converted human-readable format.
And JSON is also used much for app user-editable config despite lacking such trivial things as comments, so prevalence may signal some need, but not necessarily the need for the thing used (the awful XML is also widely used)
