Also, not all jobs come with an associated title. I'm a software engineer, but people don't refer to me as Software Engineer Joe Bloggs. "President" absolutely is used as a title. This is especially clear with former presidents, who retain the title. (E.g. George Bush, who isn't president, is still President Bush.)
Knighthood isn't bestowed on people the monarchy likes, it's given to citizens who contribute greatly and above expectations to England's civilian, military, and peacetime achievements.
Best I can tell, Sir is a title bestowed by the monarchy on people they like.