The more a format restricts, the more useful it is. E.g. if a format allows pretty much anything and it's up to parsers to accept or reject it, we may as well say "any text file" (or even "any data file") -- it would allow for anything.

Similarly to a "schema-less" DBMS -- you will still have a schema, it will just be in your application code, not enforced by the DBMS.

JSON is a nice balance between convenience and restrictions, but it's still a compromise.