I've never heard of this sport being called association football.
The term association football is the original name for the sport. FIFA's name in English is International Association Football Federation. However, its usage has diminished in recent years, with different cultures developing their own word(s) for the sport. Even the word soccer derives from the "soc" in association.
Why not just football?
This is because there are several sports that are known as football in different countries. For example, in the United States, American football is primarily referred to as football, while the same is true of Gaelic football in Ireland, Canadian football in Canada, rugby union in New Zealand, and rugby league or Australian rules football in Australia. The title association football avoids any ambiguity over which code of football is being referred to, and also removes the potential for accusations of bias towards any particular code. Meanwhile, the Football page is a "broad-concept article", providing a general overview of all of the sports named football.
Why not soccer then?
In the United Kingdom, the usage of the term soccer, a term which originated in South East England (before being popularised in the United States), is sometimes viewed as being derogatory towards the sport, or an example of American culture being forced onto the rest of the world. Therefore, although the word soccer would be an unambiguous title for this article, there would be discontent from a large number of people who object to their word for the sport being ignored. Others point to soccer being the most widely used name for the sport in English-speaking nations – however the statistics for this are not readily available or are confusing (e.g. India is the largest country with English as an official language and refers to the game as football, but English is not the primary language for most Indians) and others where countries change their official name for the sport (as Australia have done by now referring to the sport as football, renaming Soccer Australia to Football Federation Australia and changing the local associations' names to reflect this, whilst the general populace still refers to the game as soccer).
What about "Football (soccer)" or "Soccer (football)"?
On Wikipedia, the placing of a word in parentheses in the title of an article is used as a method of disambiguation, with the parenthesized word usually being a set of which the article's subject is a part. Therefore, using either of these titles implies that football is a form of soccer or vice versa, which is not the case. Using "Association football" as the title of this article is permissible under Wikipedia's practice of using natural disambiguation in article titles.