Postgres strange conversion of the string 'now' to timestamp

It looks like postgres treats the string 'now()' the same as a call to the now() function. Why does postgres allow this?

select 'now'::timestamp;

or this?

select 'now()'::timestamp;

or even this?

select '  (  (  ))) now)('::timestamp;

It does not give this "a string is the same as a function" treatment to other functions such as clock_timestamp(). [DB fiddle](https://www.db-fiddle.com/f/4jyoMCicNSZpjMt4jFYoz5/11195)