Beer goes bad because sunlight interacts with some compounds in the hops. That's why you'll find most beer in brown bottles - it blocks (most) sunlight. Beer without a lot of hops can go in clear bottles as there isn't much to turn it skunky.
This isn't entirely true. You are right that most of the mass produced beer in America will go bad, but to say that all beer goes bad is wrong. Many styles like the big Belgian beers and many strong ales get better with age because the yeast in the beer continues to develop over time.