> Don't get me wrong - I love the feature and use it a lot, but I find the syntax awkward and honestly just... strange (though I've not researched it and have no idea of the reasoning behind it).

I'm pretty sure the syntax was just reused from hstore (https://www.postgresql.org/docs/current/hstore.html), for familiarity and simplicity. hstore predates json by 4-5 years.

It might also have made expressions easier to parse, as it doesn't conflict with normal identifier access.