I'm not disagreeing that JSON is used for data serialization, but your last comment just muddies the waters. There are plenty of things that are serial in nature, but have nothing to do with "serialization" in the sense of data marshalling. I guess that the term is kind of unfortunate.

Consider just how how many data formats are ultimately defined as a "serial stream of characters" -- and then consider how few of those you would practically use for marshalling a general data structure.