Because it is! The liver is the largest gland, and the largest organ of the human body. How can I answer this question, WHY is it called that? Its size and its size relative to other organs is a fact. The question WHY is it called that is like asking why is this man called 'tall' when the person referred to is six foot four. The normal weight of a liver is getting on for 2 kgs in a man.
That's some size! A gland is any group of cells that manufactures and releases substances for use somewhere else in the body. The liver is the largest gland. It makes and secretes substances. As an organ, the largest organ, it actually has hundreds of different functions, so its importance is not only its size but its many vital functions.