String literal doesn't autocast to text type

Hello!
Why string literal like 'Hello world!' doesnt automagicaly cast to text
type?

postgres=# select pg_typeof('Hello world');
pg_typeof
-----------
unknown
(1 row)

But for example literal like 1.1 automagically cast to numeric( not
float8, float4, whatever)
postgres=# select pg_typeof(1.1);
pg_typeof
-----------
numeric
(1 row)

That why we cant do the following without explicit type casting:
postgres=# select t.c||' world' from (select 'Hello' as c) as t;
ERROR: failed to find conversion function from unknown to text

but that ok:
postgres=# select t.c||' world' from (select 'Hello'::text as c) as t;
?column?
-------------
Hello world
(1 row)

or this is ok too:
postgres=# select t.c::text||' world' from (select 'Hello' as c) as t;
?column?
-------------
Hello world
(1 row)

Sure we can create our cast:
postgres=# create cast (unknown as text) with inout as implicit;
CREATE CAST
and after that we have:
postgres=# select t.c||' world' from (select 'Hello' as c) as t;
?column?
-------------
Hello world
(1 row)

But why we don't have this type cast by default in Postgres? Is there
any fundamental restriction on that or there is some reasons for that?

--
Alex Ignatov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company