I have a theory about this. I’ve been teaching English pronunciation a long time, so questions like this come up, over and over. Here’s my thinking.
We know that listening to speech is nothing like reading a written text. They are not the same methods of communication. In English, especially American English, listening requires full attention be paid to the most important parts being spoken. Since listeners cannot listen to every vowel and consonant individually then combine them, then understand them and the message, speakers are careful to acoustically stress the parts that will help the listener the most as they hear the message.
The word BEEN often plays a supporting role in speech, for example ‘I have been calling him all night” or “She had been swimming when the lightening struck”. There often is another, stronger verb in the phrase. The word ‘been’ needs less articulation for the listener to successfully understand it. And in sentences where it’s the main verb, such as “Where’ve you been?”, logic tells us that ‘BN’ means been, not bin or Ben.
The listener only needs to to hear BN in the sentences above. Try saying one: “I have bn calling him all night”. Speakers may substitute a short i or a short e to replace the EE in BEEN—whichever sound allows the speaker to move from B to N the easiest.
And Americans reduce the number of sounds even further, streamlining the message to its least number of sounds needed by listeners to process the message. “I have” shrinks to “I’ve” because listeners only need to hear a V sound after “I” to recognize that “I’ve” was spoken.
English speech is spoken so listeners can catch the message, not so listeners can transcribe each consonant and vowel individually, string them together, and finally read them as words to discover what has bn said.
American English, especially, plays down unnecessary sounds and highlights those that are fundamental to understanding the overall message. Our spoken language is a very economical version of our written language.
The word SEEN, however, plays an important role as the active verb in messages. In this sentence, “I have seen that movie already”, SEEN cannot be reduced to sin or sen because it is not a supporting word, it is a core word that carries the meaning of the message and therefore must be articulated clearly or the message will be lost.