Firstly, because Jehovah has assured us he is the grand Creator. (Psalm 100:3) Jehovah has proven his Godship by acting throughout history to bring his purposes about. Jehovah is the God of true prophecy, "the One telling from the beginning the finale, and from long ago the things that have not been done. the One saying, ' my own counsel will stand and everything that is my delight I shall do.' (Isaiah 46:9,10)
I believe Jehovah has supplied abundant evidence of his Godship, in contrast to the gods who cannot prove they exist. The gods of Egypt, baal, Babylon's gods have all been defeated by Jehovah and consigned to the dustbin of history, barely even recognized today. Psalm 115 contrasts Jehovah with the idols of the nations. Although having a mouth, eyes, ears, hands and feet, these man-made lifeless gods cannot speak or see or walk. Meanwhile, "everything that (Jehovah) has delighted to do he has done." (Psalm 115:3) My study of the Bible convinces me and millions of others that Jehovah is the grand Creator, and the only true God.