Test this API call:
https://en.wikipedia.org/w/api.php?action=query&format=json&formatversion=2&generator=categorymembers&gcmtitle=Category:February&gcmnamespace=1
Notice that there is no "query" field in the response. This means that the struct automatically generated cannot just assume the query field exists.
While it is easy to wrap that in an Option, I believe such a change would break every existing code so this is not an easy task.