Why as the level of college course goes up, the course will generally to be more specific and more specialized?

Why as the level of college course goes up, the course will generally to be more specific and more specialized but not necessarily more difficult and not necessarily the easier one on the lower level, also you don't have to have the knowledge of the previous class even if it served as a prerequite?

For example you study the general biology first, then study anatomy or endocrinology, but you probably won't need to have the knowledge of general biology to study anatomy or endocrinology

Is there for any reason that every subject does appear to be that pattern?