Why Does the Army Call NCOs "Sergeant" and Officers "Sir"?

Because if you called an Army Officer, "Sergeant" it would confuse him for months!

SFC (Join to see), I don't know its origin, but it is regulation. AR 600-20, table 1-1 provides the titles of address (e.g., Sergeant First Class = Sergeant), and AR 600-25 provides the appropriate greetings (e.g., "Good morning, Ma'am."). *EDIT* In case you weren't aware, the title of address for Master Sergeant is "Sergeant", not "Master Sergeant."

Close answers...read my book Sovereigns, Dynasties, and Nobility available on Amazon. As with many things in history, much depends on your chosen snapshot in time. By the time of the 11th Century, "Sire" was the term generally offered to most male nobles in Europe including serf-knights. The term eventually morphed into "sir" and was applied to the merchant class and gentry. People who model their beliefs on Hollywood notions thus link 'sir" as in Sir Lancelot; however, the correct term was "sire". By the 16th Century, military officers still tended to be from noble/upper class families and were customarily referred to as "Sir".
Similarly, "Ma'am" or "Madame" was a term originally applied to noble or royal ladies (Baroness and higher).
The position/rank/term of "Sergeant" also had different meanings in different times, from Sergeant Knights/at arms (Sergeant at arms) to Sergeant Major Generals....in such cases the "title" or "rank" was applied in cases when the term "sir" or "sire" did not apply, as in the case of lower knights and squires.
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