Why do Americans call it soccer when everyone else says football?

The word soccer was invented in England. In the 1860s several football codes were being formalized, and the two that mattered most were rugby football and association football, named for the Football Association that codified it. English public school and university slang of the era added -er to shortened words, producing rugger for rugby and, from assoc, soccer. The term was thoroughly British and stayed in common British use well into the twentieth century, appearing in newspapers, book titles, and the speech of players.

American usage adopted soccer for the obvious practical reason that football was already taken by the gridiron game. The reversal came later: British usage began abandoning soccer in the second half of the twentieth century, and researchers who have tracked the shift attribute much of it to a growing perception of the word as an Americanism worth rejecting. So the common accusation has the history exactly backward. Several other countries with their own dominant football codes use the term for the same reason America does, including Australia, Canada, Ireland, and South Africa, which makes the American usage unremarkable rather than uniquely stubborn.