Many people look at a large, ornate religious building and believe it is a cathedral. It may very well be one, but not for the reasons they think. Cathedrals may be plain churches with little embellishment. The only reason a church is ever listed as a cathedral is because of its place in the hierarchy of the diocese. Roman Catholic Church dioceses are run by a local bishop. His chair in the sanctuary is a throne. In Latin, this is called a cathedra. Translated into English, it becomes a cathedral.
While it is not necessary for the administrative church of a diocese to be ornate, many of them are. This has been out of respect for the office of the bishop. Where he rules, Catholics have been honored. They showed this by ensuring his church was the most ornate in the diocese. That is why many of the most ornate churches are cathedrals.