וכלהו פרחין מן רקיע, ודא צדיק חי עולמים דמניה פרחין נשמתין דצדיקיא ונהרין בסיהרא They [the souls of the tzadikim] all fly from the RKIA; and this is Tzadik Chai-Olamim (The Tzadik, Life of the Worlds), as from him fly/blossom the souls of the tzadikim, and they shine in the moon (SYHRA)...

Now, for your question in particular, it seems the answer is because the Torah comes to teach us more in this fashion; there's a lot going on with the two MARoT; and much commentary about it, for instance I:20a

"And Elohim made the two great luminaries" (Gen. 1:16).
"And...made", וַיַּעַשׂ, signifies the greatness and the full establishment of everything, as it should be. The phrase: "The two great luminaries" indicates that the two great luminaries, (which are Zeir Anpin and his Female principle) were united at first. This is the secret of the full name Yud Hei Vav Hei - Elohim, which although not openly revealed can be surmised. So the "two great luminaries" appear in a full name, even though the name Elohim shines only in a concealed way. The word "great" means that Zeir Anpin and his Female principle grew in greatness by this name as equals, so much so that they were called by the name that unites everything together, which is Mem-Tzadi-Pe-Tzadi Mem-Tzadi-Pe-Tzadi. Zeir Anpin received his greatness from the right one, his Female principle from the left one. In such a way, they are referred to as "the two great luminaries." These are the most supernal names that precede the thirteen attributes of Mercy. The names Mem-Tzadi-Pe- Tzadi Mem-Tzadi-Pe-Tzadi are called "great," because they became great and ascended, since they are supernal and derive from the secret of above. They benefit the world, as the worlds' existence depends on them. Similarly, the two luminaries that appear in the verse, which are Zeir Anpin and his Female principle, both rose as one, in one greatness.

The moon was not at ease with the sun because she felt embarrassed before it. The moon said to the sun: "Where do you feed your flock? Why do you make your flock to rest at noon?" (Shir Hashirim 1:7). This is as if to say: How can a tiny candle shine in the middle of the day? "Why should I be like one who cloaks himself?" (Ibid.) That is: how can I remain in shame? Then she lowered herself to be head of the lower beings, as it is written: "Go your way forth by the footsteps of the flock" (Ibid. 8). Thus, the Holy One, blessed be He, said to her, 'Go and diminish yourself.'

From then on, she had no light of her own. She had only the light she received from the sun. At first, they were united as one on the same level, but she later lowered herself from all of its grades. This means that it lost all her grades and emerged from the world of Atzilut. Although she is the head of the lower worlds of Briyah, Yetzirah and Asiyah, she is nevertheless considered to have lost all her grades, as in principle a woman has no significance unless she is together with her husband. ...

And there's much more.

וכלהו פרחין מן רקיע, ודא צדיק חי עולמים דמניה פרחין נשמתין דצדיקיא ונהרין בסיהרא They [the souls of the tzadikim] all fly from the RKIA; and this is Tzadik Chai-Olamim (The Tzadik, Ever Living or Life of the Eons), as from him fly/blossom the souls of the tzadikim, and they shine in the moon (SYHRA)...

Now, for your question in particular, perhaps it's because the Torah comes to teach us many things through it; there's a lot of commentaries on the two מארת חסר such as being without Waw they are kind of cursed, i.e. male and female are rooted here, from RKIA; similarly we have לא טוב היות האדם לבדו. Maybe you will find more answers in the below, or someone can add/improve.

For instance I:20a

"And Elohim made the two great luminaries" (Gen. 1:16).
"And...made", וַיַּעַשׂ, signifies the greatness and the full establishment of everything, as it should be. The phrase: "The two great luminaries" indicates that the two great luminaries, (which are Zeir Anpin and his Female principle) were united at first. This is the secret of the full name Yud Hei Vav Hei - Elohim, which although not openly revealed can be surmised. So the "two great luminaries" appear in a full name, even though the name Elohim shines only in a concealed way. The word "great" means that Zeir Anpin and his Female principle grew in greatness by this name as equals, so much so that they were called by the name that unites everything together, which is Mem-Tzadi-Pe-Tzadi Mem-Tzadi-Pe-Tzadi. Zeir Anpin received his greatness from the right one, his Female principle from the left one. In such a way, they are referred to as "the two great luminaries." These are the most supernal names that precede the thirteen attributes of Mercy. The names Mem-Tzadi-Pe- Tzadi Mem-Tzadi-Pe-Tzadi are called "great," because they became great and ascended, since they are supernal and derive from the secret of above. They benefit the world, as the worlds' existence depends on them. Similarly, the two luminaries that appear in the verse, which are Zeir Anpin and his Female principle, both rose as one, in one greatness.
The moon was not at ease with the sun because she felt embarrassed before it. The moon said to the sun: "Where do you feed your flock? Why do you make your flock to rest at noon?" (Shir Hashirim 1:7). This is as if to say: How can a tiny candle shine in the middle of the day? "Why should I be like one who cloaks himself?" (Ibid.) That is: how can I remain in shame? Then she lowered herself to be head of the lower beings, as it is written: "Go your way forth by the footsteps of the flock" (Ibid. 8). Thus, the Holy One, blessed be He, said to her, 'Go and diminish yourself.'
From then on, she had no light of her own. She had only the light she received from the sun. At first, they were united as one on the same level, but she later lowered herself from all of its grades. This means that it lost all her grades and emerged from the world of Atzilut. Although she is the head of the lower worlds of Briyah, Yetzirah and Asiyah, she is nevertheless considered to have lost all her grades, as in principle a woman has no significance unless she is together with her husband. ...

There's much more in Zohar and TZ and the other Sifrei Kodesh. For me sifrei Breslev and TZ are easiest to understand.