If wasting seed is so severe, why is it not explicitly mentioned in the Torah?

With G-d's help 

Hello and blessings, Our sages did not learn the prohibition of wasting seed from the story of Er and Onan (this story may serve as a peripheral proof of the serious lack of modesty involved in this act in the eyes of G-d, understanding that Onan's added intention not to bring children into the world led to a decree of Karet for him in this world). The heart of man is drawn to the prohibitions of forbidden relations, constantly seeking loopholes and excuses in this matter. However, the Holy Torah forbids one even to look at and think about women: "And do not stray after your hearts and after your eyes that you go astray after them, so that you remember and do all My commandments and be holy to your G-d" (Numbers 15). If the Blessed Creator forbids looking at women and even prohibits thinking about women, is it conceivable that the act of wasting seed could be permitted? This is a kal v'chomer (a fortiori argument). It is needless to say that one who wastes seed thinks about women and looks at them, and all his thoughts and eyes are tied to fornication and lust and pornography, and then he deceives himself into believing that he fulfills the will of the Holy Torah which commands "And you shall be holy to your G-d"? "Do not come near to uncover nakedness; I am Hashem" (Leviticus 18:6). "I made a covenant with my eyes; why should I look at a maiden?" (Job 31:1). "Speak to all the congregation of the children of Israel and say to them: 'You shall be holy, for I, Hashem, your G-d, am holy'" (Leviticus 19:2). And anyone who believes in the Holy Torah must believe in all that our sages transmitted to us in the Oral Torah. As is known, our sages compare the act of wasting seed to spilling blood, and it is ruled in the Shulchan Aruch that this is a very grave sin. Any person with a sound mind understands that this sin distances a person from the Creator very much, as it is entirely a preoccupation with one's body and desires and self-centeredness for no purpose. For the entire purpose of intimate relations is for the mitzvah of "be fruitful and multiply" as well as the wonderful and sacred connection between man and wife: "And a man shall cling to his wife, and they shall become one flesh" (Genesis 2:24). One who spills his seed for no purpose acts against the purpose of creation; he uses the organ that Hashem gave him for sin instead of for a mitzvah, and instead of the wonderful connection with a woman out of love for her, he uses his procreative organ for selfish, physical desire. Indeed, the opposite of the sacred relationship between man and woman is the selfishness involved in wasting seed. The fact is that one who engages in wasting seed - becomes addicted to the sight of women, learns to look at women as objects for personal gratification. He disdains his own body and disrespects the marital system with a woman. This desire causes the brain to become accustomed to thoughts about various women, which complicates married life and family peace. Like any drug, the dosage continues to rise, and the pornographic materials become increasingly difficult, as he cannot enjoy the lighter materials that he once enjoyed - which of course also affects his future performance with his wife. A young man who becomes accustomed to this does not learn to control his impulses, which harms the quality of his life. It is a statistical fact that the most successful men are those who can control themselves. This will manifest in all areas, including family life and parenting. Those who engage in this know very well that wasting seed drains their energy and exhausts them physically and mentally. It is a fact that students, as well as athletes and wrestlers, do not waste seed during their study/training periods, because it negatively impacts their academic and motor performance. Young men who have succeeded in overcoming the act of wasting seed report that the first thing they feel after a week is a strong sense of mental strength, self-confidence, and happiness. Those who engage in it often suffer from depression and fatigue. This is where the hunger (eating and tiredness leading to sleeping) comes from after wasting seed, as the body loses energy and proteins. It has been scientifically proven that after wasting seed, the brain immediately releases substances that cause a person to feel emotional closeness - intended to strengthen the marital bond between a man and his wife. When not with a woman, a person will feel a sense of loss and guilt because he has no one to bond with emotionally. It is no coincidence that it is called "for no purpose." And most importantly: This action distances a person from serving Hashem and correcting his character traits, as it causes a person to focus on himself and his physical desires. Overcoming the evil inclination does not happen overnight, but rather it is a process of rising spiritually in stages, and the main thing is for the person not to despair and not to hate himself. 

Please read in my articles: 

 https://www.hidabroot.org/article/2638 

 https://www.hidabroot.org/question/99304 

 With blessings, Daniel Bles