Psalm 2 v 7 I will declare the decree: the LORD hath said unto me, Thou art my Son; this day have I begotten thee.
God never actually spoke these words to David. He only spoke them to Jesus because only Jesus was the only begotten Son of the Father when he came in to the world.
These words may have been written to or about David in some sense but God never spoke these words to David face to face. Even as a king he is simply appointed and anointed by God to rule, but he is not begotten as if there is a special process involved. To claim that David is begotten is to devaluate the exclusive meaning of the word as it applies to Jesus alone. God the Father has never spoken these words to anyone except to Jesus alone.