Basically all dress shirts have four corners - at least. Their are usually two corners in the front bottom and two clear sharp corners on the edges of the collar. Why do we not put Tzitzit on these dress shirts? I have heard that there is a doubt whether a shirt with four corners that are on the same side qualifies for Tzitzit, however, in that case, there is still a doubt that perhaps it does qualify for Tzitzit then Tzitzit should either be put on it or some of the corners rounded.
The idea that if the four corners are on the same side, the garment will not require Tzitzit is not a doubt, but the opinion of the Rema. And even though his reason is not accepted among all other Poskim, nevertheless there are many other reasons in the Poskim why a dress shirt does not require Tzitzit. The following are some of the reasons: A garment requires Tzitzit only when the corners stick out clearly. The corners of the shirt lost their shape due to the way it is sewn. A garment that has sleeves is not considered a four cornered garment that requires Tzitzit according to the Torah. The corners of the collar are not considered corners regarding Tzitzit because the collar is folded over. See at length the Tshuva of the Tzitz Eliezer. [שו"ע ורמ"א או"ח סימן י סעיף יב ומ"ב ס"ק לו, ערוך השולחן סימן י סעיפים טו-כ, חזו"א הלכות ציצית סימן ג ס"ק לז לענין צוארונים, ציץ אליעזר ח"ו סימן א, ספר ציצית של הרב צבי כהן סימן י סעיף יב הלכה פסוקה סעיף נב ובהערה 70]