Why do they call soccer football in Chile?

The game's true name is, "association football". In formal circles that is how it is referenced.
However, in various countries around the world it is called, 'soccer'. This is the case in regions where other football codes are the more locally popular code - especially in countries which have formed their own brands - such as US and Canada, where they use the term exclusively for American/Canadian football. In Australia, Australian football is called football; therefore association football gets called, 'soccer' there. It is also called soccer in Ireland, New Zealand, parts of Africa, etc.