IF NOT EXISTS - seems to be applied at random in postgres

there is a mechanics to CREATE or DROP objects only whether they exist or not. To me it seems that postgres applies this randomly.

CREATE EXTENSION > true
CREATE SCHEMA > true
CREATE DATABASE > false
CREATE ROLE > false

I am aware that you can build something in pl/pgsql or create a function mimicking this behavior, which requires quite extendet skills. For my understanding ... is there any explanation why this is setup in this 'random' manner?