A soccer field is called a "pitch" because the British definition of the word, according to the Oxford Dictionary, is "a playing field." Soccer was created in England, and while it has become popular among U.S. soccer fans to adopt the term, it is simply the British version of "field."
In England, the term "pitch" is also used to describe the field used to play cricket and rugby, according to Sporting Life 360. The English consider the word "field" as describing an open area of land used for farming. Therefore, the term "pitch" is a more specific term for an area of land dedicated to sport. In England, soccer reigns as the most-watched sport, and as the country that founded it, British terminology is normally the default.