American Beer is beer produced in America. Prohibition of alcohol in America during the early 20th century led to the demise of nearly all breweries in America. Those that survived by making root beer, converting to dairy processing, or bootlegging made beer as cheaply as possible for many reasons. Among them are a poor economy, catering to a public now unaccustomed to flavorful beer, and having to meet a high demand very quickly. Adjuncts such as corn and rice became commonplace. A rich and colorful brewing tradition of hundreds of years was almost lost. The ensuing race to mass produce flavorless swill led to the rise of the "Big 3", whose weak and watery pilseners are the butt of many old, obsolete, TIRED jokes. The fact is, that if one doesn't realize that there is a greater wealth of creativity, experimentation, flavor, body, and style in America's brewing industry right now than in much of the rest of the world, well; one is simply not paying any attention.
If you think all American beer sucks, you probably either don't get out enough or just don't like beer.