Why aren't the planets mentioned in the Genesis 1 creation account?

Because there are a lot of things not mentioned in the Genesis 1 creation narrative: comets, asteroids, supernovas, galaxies, quasars, and even air. Thomas Aquinas noted that Genesis 1 mentions light, earth, and water but not air, one of the four Aristotelian elements, and he explains how Moses implied it by the darkness upon the face of the deep. God had a limited number of words; the creation is so complex he could have gone on for books, but he did it in about one page. That teaches us a lesson about how to interpret Genesis 1: the things not mentioned there are given elsewhere. One of the greatest books on creation is Isaiah, with snippets of creation material in dozens of places, and you also find it in the Psalms, Ecclesiastes, Proverbs, Jeremiah, and especially Job. This is what I discovered working on our film series, How the World Was Made in Six Days: most of the information is not in Genesis 1, which only gives a hint here and there, so you have to piece it together carefully from all these passages, reading the actual Hebrew, especially of Isaiah, to get the full picture. So it is no surprise Genesis 1 does not mention planets, since it does not mention many things out in the cosmos.