Why do all martial arts logo's look the same?

You have to realize that these logos are a western practice, mostly in the U.S. It's designed to attract customers by evoking a feeling of familiarity with martial arts. So they try to stay as close in design to the logos of other popular schools depicted by western media. In their country of origin, like China and Japan for example, most martial arts schools use Calligraphy or Kanji to depict their school's logo, and it's usually printed on their uniforms. But since most foreigners can't understand what these characters mean, most western schools have to design a logo with English translations in them or sometimes with pictographs of the character's meaning, like Dragon, Tiger or the YinYang symbol of Tai Chi for example, to make them understandable to English speakers, so they try to fit all those words and pictographs in a small logo's space as best they can. Since a circular logo is more economical and easier to design compared to other shapes, it became the most frequently used design when it comes to patches printed or sewn into a uniform, although a triangle comes a close second.