The U.S. calls football soccer because the word started in England, not America, as a slang short form of “association football.” When American football became the dominant local sport, the U.S. kept “soccer” to avoid confusion, while many other countries kept “football” for the global game.

Where the word came from

In the 1800s, English schools used playful shorthand and “association football” was eventually clipped to “soccer.” The name was already in use before the U.S. made American football its own major sport.

Why the U.S. kept it

Once American football grew popular in the U.S., “football” usually meant the gridiron game, so “soccer” became the practical label for association football. That naming split stuck through habit, media use, and sports culture.

Why the rest of the world says football

Most countries use “football” because that sport became the main code associated with the word, while the U.S. had another football already. So the difference is mostly about history and local naming, not different sports.

TL;DR: “Soccer” is an old English nickname for association football, and the U.S. kept it because “football” already meant a different sport there.