Very good question.
Our ancestors relate to age, as with time in our culture, Mother Earth and everything on and around her as wisdom.
A grandmother or grandfather that has lived the culture and teaching of the ancestors are wise with age.
Since Creator created everything including grandmother and grandfather and grandmother and grandfather cannot create Creator, it would therefore make perfect sense to call Creator Grandfather, would it not?
Welcome to the city Seattle Washington
The city Ashburn Virginia
Why do Native Americans call their god the grandfather
Grandfather (Ojeesan, Ohe-he-shun) a name for God. 8/4/11
One has no need to make things complicated when things are simple?