Actually, they don't. The true OT saints understood and knew that the coming Messiah would be God in the flesh:
God, who at sundry times and in divers manners spoke in times past unto the fathers by the prophets, hath in these last days spoken unto us by His Son, whom He hath appointed Heir of all things, by whom also He made the worlds, who being the brightness of His glory and the express image of His person, upholding all things by the word of His power, when He had by Himself purged our sins, sat down at the right hand of the Majesty on high, being made so much better than the angels, as He hath by inheritance obtained a more excellent name than they.He said unto them, "How then doth David in the Spirit call Him `Lord,' saying, `The LORD said unto my Lord, "Sit Thou on My right hand until I make Thine enemies Thy footstool"'? If David then call Him `Lord,' how is He his son?"For unto which of the angels said God at any time, "Thou art My Son; this day have I begotten Thee"? [Psalm 2]
And again, "I will be to Him a Father, and He shall be to Me a Son"? [2 Samuel 7]
And again, when He bringeth in the First-Begotten into the world, He saith, "And let all the angels of God worship Him." [Deut. 32:43, Psalm 97]
And of the angels He saith, "Who maketh His angels spirits and His ministers a flame of fire." [Psalm 104]
But unto the Son He saith, "Thy throne, O God, is for ever and ever; a scepter of righteousness is the scepter of Thy Kingdom. [Psalm 45] Thou hast loved righteousness and hated iniquity; therefore God, even Thy God, hath anointed Thee with the oil of gladness above Thy fellows." [Isaiah 61]
And, "Thou, Lord, in the beginning hast laid the foundation of the earth, and the heavens are the works of Thine hands. [Psalm 102]
They shall perish, but Thou remainest; and they all shall wax old as doth a garment. [Isaiah 34] And as a vesture shalt Thou fold them up, and they shall be changed. But Thou art the same, and Thy years shall not fail."
But to which of the angels said He at any time, "Sit on My right hand until I make Thine enemies Thy footstool"? [Psalm 110]
Are they not all ministering spirits, sent forth to minister for those who shall be heirs of salvation?
Furthermore, God is already at work in Genesis 3, preaching the coming of His salvation to the Church. The OT is one string of prophecies after another about the glorious Mighty Work of Redemption with which God will reclaim His own forever. A verse of my own meditation of the last few weeks:
Sing, O daughter of Zion! Shout, O Israel! Be glad and rejoice with all the heart, O daughter of Jerusalem! The LORD hath taken away thy judgments; He hath cast out thine enemy. The King of Israel, even the LORD, is in the midst of thee; thou shalt not see evil any more. In that day it shall be said to Jerusalem, "Fear thou not"; and to Zion, "Let not thine hands be slack. The LORD thy God in the midst of thee is mighty; He will save, He will rejoice over thee with joy; He will rest in His love, He will joy over thee with singing."