They're called master bedrooms because they are in general the largest of the bedrooms in any home design, and were created typically for the "Master" of the house. A term that dates back hundreds of years.
The main bedroom in a house is called the master bedroom because it is the room where the master of the house sleeps. Although this term is no longer commonly used, it was once normal to refer to the owner of a home as the "master" of it.
The master bedroom is usually occupied by the master, or masters, of the household. There is no requirement stating that it must be larger than the rest of the bedrooms, but that is usually the case.
On the countries such as North America, Australia, Europe have a separated bedroom called master bedroom. Generally it is dedicated to heads of the household i. e. , husband and wife or parents.
