Because if God ever had a beginning, then before He began there was nothing. Now nothing, with nothing to work upon, and no faculties with which to work, could never turn its non-existent self into something. But there is obviously something, and there can never have been a time when there was nothing. God at least must always have existed, and if no one is responsible for His beginning, there is no one who could possibly bring His existence to an end. He always will be. God rightly declared Himself the eternally existent Being when He said to Moses, “I am Who am.”
Nature of God, Question 9 from Radio Replies Volume I Copyright © 1938 (by Rev. Dr. Leslie Rumble, M.S.C. and Rev. Charles Mortimer Carty).