Unitarians take the position that the Father is the default referent of Yahweh or Elohim in the OT. I’ve discussed this contention before, but now I’d like to approach the issue from a different angle.
One of the major themes in the Fourth Gospel is the Son’s role as the revelation of the Father. By knowing the Son, you come to know the Father as well. The Son uniquely mediates knowledge of the Father. Other NT passages pick up on the same theme.
If, however, OT references to God single out the Father, then the Jews already knew the Father from OT revelation. They had direct knowledge of the Father.
So why would we need indirect knowledge of the Father via the Son? On the unitarian view, knowing the Son doesn’t contribute anything unique to the knowledge of the Father we can get straight from the source in the pages of the OT. Indeed, on a unitarian view, knowing the Son would be a superfluous and inferior means of knowing the Father, compared to the immediacy of OT revelation.