Question about casts

Just out of curiosity (and most likely, ignorance). Why can't I cast an 
array of strings into a string? I.e.
 thhal=# select ('{"a","b"}'::varchar[])::varchar; ERROR:  cannot cast type character varying[] to character varying

or a cstring into a varchar, i.e.
 thhal=# select array_out('{"a","b"}'::varchar[])::varchar; ERROR:  cannot cast type cstring to character varying

ISTM, the implementation of such casts should be fairly simple and 
straight forward and sometimes even useful. Every data type comes with 
string coercion routines anyway right?

Regards,
Thomas Hallgren