The commandment to attach tzitzit to the four corners of our garments is a Torah obligation, as per Bamidbar 15:38 and Devarim 22:12.
Our Sages of blessed memory ruled that a garment that does not have four corners is exempt from the obligation of tzitzit.
Later, when the four-cornered garment started becoming a less-popular mode of clothing, Jewish males began wearing a special four-cornered garment known as the tallit katan in which to attach tzitzit to, so the mitzvah would not go extinct.
My question is thus: what is considered a four-cornered garment? I assume something that has four corners along the bottom, but what about dress shirts? Technically speaking, they have two corners at the bottom hem, two corners at each cuff and two corners at the collar. Why are those corners on the cuffs and collar exempt from tzitzit?