as far as I can tell, it's cause they are hardcore JS devs, where JSON is seen as almost a part of the language, and little to no effort was put into things like future-proofing. A lot of the failings have been fixed as the community grows, but this one is core to how the whole thing works, so changing it at this point is fairly non-trivial (would basically involve completely breaking any kind of backwards compat, and potentially even some forwards compat)

probably they did that, then Roy Fielding came in, unplugged their router and wouldn't let them back on the internet til they stopped polluting their resources with metadata