Because it is the symbolic wild beast described in Revelation, chapter thirteen, verses one to eight, concerning which Re 13 verse two says: “And the dragon gave to the beast its power and its throne and great authority.”
... That is why Revelation, chapter thirteen, pictures this symbolic wild beast as having seven heads, to represent seven world powers in succession, namely, (1) Egypt, (2) Assyria, (3) Neo-Babylonia, (4) Medo-Persia, (5) Greece, (6) Rome, and (7) the dual world power of Britain and America.