Why does to_json take "anyelement" rather than "any"?

to_json is declared as taking "anyelement" as input, which means
you can't pass it something of unknown type:

postgres=# SELECT to_json('foo');
ERROR: could not determine polymorphic type because input has type unknown

But this works fine with the very similar json_build_array function:

postgres=# SELECT json_build_array('foo');
json_build_array
------------------
["foo"]
(1 row)

The difference is that json_build_array takes type "any" as input, while
to_json takes "anyelement" as input.

Is there some reason to_json couldn't be switched to take "any" as input?
Hacking this together seems to mostly just work:

postgres=# CREATE FUNCTION my_to_json ("any") RETURNS json LANGUAGE 'internal' AS 'to_json';
postgres=# SELECT my_to_json('foo');
my_to_json
------------
"foo"
(1 row)

Is there something I'm missing?

Nikhil