"Omni" for a certain definition of "omni" after all.
At a certain point you have to expect that he chooses to do it. After all, in Genesis (lmao autocorrect tried to suggest "Genshin Impact") creating the world took God 7 days (well, 6 days + 1 day to rest and admire). Surely someone omnipotent could've done it quicker - so it's either that or he simply chooses to take longer. ;)