Honestly? I feel like Oda just did that to give lil' baby Shanks "amnesia" so he wouldn't remember any of this. God Valley. Being a Celestial Dragon. None of it. That way, he'll be surprised later when Roger tells him his true lineage.
...I don't think that was necessary. Because, again...he's a baby. He probably wouldn't have remembered any of this, anyway. But, I think this is what Oda was thinking about. Making sure fans didn't complain about a potential plot hole.
And maybe it's partially to make Shanks a little more like Goku from Dragon Ball. Because Oda is a big fan of Dragon Ball. A nice tip of the hat to a old mentor, as it were.