Email recipients move to cc instead of remaining as a primary recipient

Why do primary recipients move to cc when replying to all? How is it possible that Apple has no understanding of how email etiquette works? CC means you are not obligated to respond or are included as an FYI. There could be more than one person expected to reply to an email and those would be kept in the primary recipient field. Many people filter CCs to a separate folder they check "when they have time", so they don't even see the message until they decide to check that folder. This makes it impossible for me to use your email app for business, even though every other feature is better than the competitor apps. I don't want to waste time shifting from one field to the other every time I reply.