is any reason why we cannot cast from record (row) to typed row?

Hello

I looked on question on
http://stackoverflow.com/questions/2104811/execute-using-statement-in-pl-pgsql-doesnt-work-with-record-type

I was surprised so isn't possible cast from record to target type - is
there reason for this?

DECLARE r RECORD;
BEGIN EXECUTE 'SELECT * FROM xx' INTO r;
 r::xx isn't possible r::text::xx is possible

Regards
Pavel Stehule