They probably aren't, at least at the level we often teach. Much of our knowledge about the brain comes from observing people who have pieces missing and seeing how their behaviour differs from a normal adult or putting people in an fMRI scanner and saying "wow, that area used a lot of oxygen compared to baseline". This, and a scientist's nature to classify things, led to a lot of overoptimistic categorization of brain function to specific regions. As neuroscience has matured the field has grown to recognize a more nuanced view that most computation in the brain is more distributed than we first assumed, and different areas are often involved in overlapping functions. It can also change over time or after extreme brain trauma. But it's not correct to say it's fully distributed either. The honest answer is we still have an extremely poor understanding of how the brain works.