Quite simply, because they are partly hidden, like real Easter eggs, and if you find and open one, you will find a goodie, a treat or a surprise inside. http://www.kidsites.com/sites-fun/vgames.htm

They are called "easter eggs" after the easter tradition of hiding eggs with prizes for the kids. Finding the egg is supposed to be challenging for the kid, and finding it of course means a reward. The case in the "easter eggs" hidden in the different media is the same, they are supposed to be difficult to find and of course finding them should represent a reward (spiritual, of knowledge, or just recreational). These hidden so-called "easter eggs" can have a multiple variety of purposes, determined by the believes, position and nature of the person that hide them. Not everytime is the author or producer the responsible, but sometimes somebody who participated directly or indirectly in the process of creation. The "why" is something that only the author can answer, but here are some examples...without citing the medium: -Some are just the made of pranksters, like a computer programmer that hides his favorite quote, an insult or just a popular saying inside a program. -Some can be a rebellious message, like a politician in jail that writes a book about nothing relevant, hiding his/her true thinking embedded in a code thru the pages. -Some can be just commercial, trying to make the buyers of a particular book to visit a webpage that gives you hints about something hidden on a cd, book or dvd. -Some are just recreational, giving the buyer or user something funny to do besides the main purpose of his/her purchase. -Some can be a actual means of communication, like the objective and set of rules of a secret society which can only be found if you belong to it and have the knowledge to decode it. -Some can be intended as subliminal, like in the case of a rock band that inserts a message in his music or a film producer that inserts a word or words in the screen. The "eggs" are not always only symbols, they can be complete messages (written, sung, video recorded, drawn or programmed). Sometimes you have to decode things, or put together different pieces or play music faster or slower, play a video single frame by frame, take the first letter of every chapter on a book, or see something against a mirror, even hold a set of keywords in a program to make the "egg" execute. There are lots of people who called themselves "egg hunters" and devote time to study of analize particular pieces of art, or books or anything that they believe (because of its nature, or the believes of the author) are potential hiding places. Interesting, isn't it?