The idea of creating living lifeforms was done so that they can have people that will worship the world they have created.
Another intention of the ten Gods for pushing through with the creation of a world and living lifeforms is so that those created lifeforms would worship them as their creator.
With that in mind, whatever it was they were going to be creating had to be done by themselves, with their efforts and essence.
To achieve the goal of having whatever lifeform they created revered them as their Gods, they had to by themselves, bring the created lifeform to life.
Just as how a mother gives birth and through that childbirth, imprints her mark on the child as her own. Or how one buys a commodity registered under their name.
True, the commodity is not necessarily created by the buyer, but that is not the point. The point here is; before the buyer can claim the commodity as their own, they released their name and had it registered under it.