At this point in time when this text was created, there was a temptation to worship divinities in the sky.

This was a particular temptation for the Egyptians (who play a large part in the whole of the Old Testament).

This temptation was also present for God's chosen people.

So in describing the creation of the heavens, where we have the heavenly bodies God's chosen people might be tempted to worship, the author of Genesis demotes them in a way to counter that temptation.

He demotes them by not calling them good.

At this point you might wonder, is there any other creation not called good?

There is indeed, and you might be surprised to know that part is....

...Human Beings!

To be clear, there is something special about them because they're created in the image of God, but they're not called good.

What does that mean?

Well, it doesn't mean they're not good. Everything God creates is good.

What we want to ask is why the text omits calling them good.

To understand this, we need to notice that in this creation account, each time God speaks, He creates something that has a greater degree of motion, a greater degree of freedom from being "locked into place" so to speak.

By the time we get to human beings, they have the greatest motion. To be clear: heavenly bodies are limited to a fixed orbit, plants are limited to one place but can move by developing upward, animals can move from place to place but they're fixed in their instinct so there's really only one path they can follow. You get the idea.

Human beings though can not only move around but can move on many different paths; they can surpass instinct; they don't have just one path they can follow through life (en contra to the animals). Human beings have a kind of freedom from place in order to find their place in God's creation.

In other words, there's something incomplete about human beings; we're not yet perfected in our freedom.

Because all the other things in creation are good and complete, those can be called good. But the human being is yet not fully good because we have to grow into God's image and likeness.

We have to grow into the fullness of what we're meant to be.

It wouldn't be entirely wrong to say the rest of the Bible is about human beings trying to do this in all other ways aside from God.

P.S. Some of the Church Fathers believed the Magi to have been star-worshippers too, and that God may have have used their fictitious deities to show them the way to the true God.