becuase the symbolism referred to is the ‘work’ and the seventh day is symbolic of ‘rest’, a frame work for us to follow....
what is so hard to understand about that?
Days don't refer to work...
God could have said he first created one thing, and then he created the next things, etc...And when he was done, he stopped working...He was finished...
But God took the time to let us know what He created on each day of Six days...And the Seventh day, He rested...God certainly didn't need to go into any detail to let us know He worked and then He was finished working...
Some serious Scripture study points out why God was so specific...