The lintel is a very basic structural element, without which a doorway is incomplete and therefore exempt from mezuzah.[1]
[Practical halachah deals with lintels of unusual shape, position or purpose as well as doorways without a lintel, but whose ceiling edge can be seen as a lintel. These are discussed in other questions.]
[1] The third condition of the Rambam, cited on our site in General Principles section.