Sure it doesn’t have to be JSON. It could be YAML, XML, ASN.1, S-expressions, whatever. But I think JSON is a clear winner (nowadays) on the basis of being simple, easy to parse (compared to C/C++/etc which have very complex syntax), parsing libraries being extremely widely available (almost every language, even the most obscure, has a JSON parser available; languages which lack JSON support in their standard library, such as C, are now the exceptional minority.)
Of course it is more voluminous and slower than a binary format. One could always define a binary encoding which straightforwardly maps to the JSON one (or just reuse an existing one such as UBJSON, BSON, CBOR, etc)
