I don't see any particular reason not to add it, but there are some workarounds you can use in the meantime:
Build a cache of all site information with the site URL as the key, and then look up the api_site_parameter value (and associated information) that way.
Use the fact that the domain name can be passed as a valid value for the site parameter, in ...
See the documentation for network_user. top_answers and top_questions are not returned by default; you must specify them explicitly in a filter.
When you ran /me/associated from the doc page and from your app, you used a filter (!-*f(6r3ZbZjn) that specified those two properties.
When you used the address bar of the browser, you didn't specify a ...